Psychology
2013. Vol.4, No.6A2, 19-26
Published Online June 2013 in SciRes (http://www.scirp.org/journal/psych) http://dx.doi.org/10.4236/psych.2013.46A2004
Saliva Cortisol and Heart Rate Variability as Biomarkers in
Understanding Emotional Reaction and Regulation of Young
Children—A Review*
Ciwas Pawan, Shu-Mei Chwo, Ishien Li#
Department of Child Care and Education, Hungkuang University, Taichung, Taiwan
Received April 11th, 2013; revised May 12th, 2013; accepted June 10th, 2013
Copyright © 2013 Ciwas Pawan et al. This is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the
original work is properly cited.
Cortisol and heart rate variability (HRV) are good indicators for the non-invasive assessment of the hy-
pothalamic-pituitary-adrenal (HPA) and autonomic nervous system (ANS) activity in response to psy-
chophysiological stress respectively. Emerging evidence from previous studies suggests a link between
cortisol and HRV response to stress and social experiences during early development. However, research
in this area has been constrained by a number of conceptual and methodological challenges. Time is a
crucial variable that needs to be taken into account in study designs since stress-sensitive physiological
systems change over time in response to changing intrinsic and extrinsic states. In this review, our focus is
on the HPA axis and HRV responses as an allostatic system with young children’s individual differences
in temperament, social regulation, and environmental sources of influence taken into account. The con-
clusions include: 1) cortisol levels are related to various time courses, ranging from moment-to-moment
changes to changes occurring over the course of days, months, and years in consideration of individual
differences in state and trait emotions; 2) it is necessary to take individual characteristics, multi-faceted
constructs related to early development, and developmental changes into account in studies of reactivity
and regulation patterns of the cortisol and HRV in young children; and 3) prospective examination is
needed on the long-term outcomes of various individual characteristics and environmental influences (e.g.,
attachment quality, family and daycare environment, and environmental control of the child) in early ex-
perience that are related to reactivity differences in HRV and atypical cortisol patterns.
Keywords: Cortisol; HRV; Children; Emotion
Introduction
Measuring the activity of the hypothalamic-pituitary-adrenal
(HPA) axis is a method that is used to monitor and understand
the stress response. When the HPA axis is activated due to ex-
posure to a threat, circulating cortisotroid concentration raises
in about 15 - 30 min and returns to un-activated levels some
time after the stressor disappears or has been removed (deKloet,
Sibug, Helmerhorst, & Schmidt, 2005). The HPA system is one
of the biological mechanisms that activate the attention and
energy responses needed to face threats (Gunnar, 2001; Di c k e r so n
& Kemeny, 2004; Segerstrom & Miller, 2004). Reactivity and
regulation of the HPA system has been implicated in the etiol-
ogy of physical illness, substance abuse, and serious psychiatric
conditions such as depression, anxiety disorder, and posttrau-
matic stress disorder (Goeders, 2003; Gold & Chrousos, 2002;
Luby et al., 2002; Mathew et al., 2003; Roma, Champoux, &
Suomi, 2006).
Although cortisol (the major hormone produced by the HPA
system) provides only a partial understanding of the activity of
this neuroendocrine system, its regulation may bear importance
for human development (Gunnar & Donzella, 2002). The re-
lease of cortisol can be reliably measured in saliva (Kirschbaum
& Hellhammer, 1989) and its measure has made possible the
assessment of the immediate biological impact of the environ-
ment. In the last two decades, a considerable amount of re-
search on the regulation and dysregulation of the stress system
in young children has relied on salivary cortisol measures.
Studies on rodents and primates suggest that activity and
regulation of the stress system later in life may be shaped by
experiences during early development; stressful events trigger
immediate changes in the stress system that may permanently
alter brain functions and behavior (deKloet, Sibug, Helmerhorst,
& Schmidt, 2005; Gunnar & Donzella, 2002). Research with
human infants and young children show an association between
cortisol reactivity and learning or memory for voice/object cor-
respondence (Thompson & Trevathan, 2008) and intervention
effectiveness in improving the HPA axis regulation of indi-
viduals with early separation from caregivers (Dozier, Peloso,
Lewis, Laur e n ceau, & Levine, 2008).
Elevations in cortisol are typically the focus of research on
HPA axis dysregulation; however, it is important to note that
the system can respond bi-directionally. A wealth of research
*This work was supported by a grant (99-2410-H-241-005-MY2) from the
N
ational Science Counc il, Taiwan.
#Correspo n ding author.

C. PAWAN ET AL.
demonstrates that elevations in cortisol and dysregulation of the
HPA axis are related to physical illness, substance abuse, and
serious psychiatric conditions such as depression, anxiety dis-
order, and posttraumatic stress disorder (Goeders, 2003; Gold
& Chrousos, 2002; Luby et al., 2002; Mathew et al., 2003;
Vanitallie, 2002). Nevertheless, studies also found that cortisol
levels of young children under conditions of neglectful and
abusive care are reduced rather than increased (Gunnar & Don-
zella, 2002). Relations between hypoarousal of the stress sys-
tem and antisocial, aggressive, and criminal behaviors were
shown in adults (Raine, 2002; Susman & Pajer, 2004; Susman,
2006). Therefore, chronic elevation and depression of cortisol
are considered possible influences of long-term behavioral and
developmental outcomes.
The parasympathetic branch of ANS as measured by vagal
control has been the primary focus of research on individual
differences in behavioral or temperament-based responding to
the environmental stimuli, while the sympathetic branch of
ANS has been the focus of normative changes in response to
the environment (Stifter & Jain, 1996). Circadian vagal tone is
an index of the functional status of the parasympathetic nervous
system that has been considered as a psychophy siological marker
of emotion regulation and arousal (Porges, 1995; Porges, 2001).
Parasympathetic nervous system functioning, measured by high
frequency variability in heart period, is related to the control of
attention, emotion, and behavior. The high frequency power in
heart period is mainly a result of respiratory influences (respi-
ratory sinus arrhythmia). Porges (1995) has developed methods
for quantification of power in this frequency band and has
named it vagal tone.
Recently, measurement of HRV (also a non-invasive tech-
nique) has been widely used to investigate the functioning of
the ANS, especially the balance between sympathetic and vagal
activity. It has been proven to be very useful for both research
and clinical studies concerned with hypertension, psychiatric
and psychological disorders, cardiovascular disease, and dia-
betic autonomic dysfunction. Over the past decade, HRV has
been used increasingly in the analysis of changes in sympa-
thetic-vagal balance related to individual differences (e.g., tem-
perament and emotion regulation strategies) and psychologi-
cal/environmental stressors (Santucci, Silk, Shaw, Gentzler, Fox ,
& Cohn, 2008; von Borell et al., 2007).
Examination of the physiological underpinnings of social-
emotional regulation in childhood is valuable for understanding
individual emotional responses, self-regulation behaviors, and
social regulations under stress. To more accurately assess in-
ternal emotional states and interpret environmental influences,
physiological measures of young children’s stress system are
needed. It is difficult to distinguish between two individuals
who behaviorally respond to a stimulus in a similar manner.
Similar situations may elicit different levels of e motional a rou sa l
from different individuals. Also, due to individual differences
in the intensity of emotional reactivity, individuals may exhibit
similar levels of distress behaviorally but show evidence of
different levels of physiological arousal. Behavioral displays
maybe mislabeled or mi sinterpreted without physiological m eas-
ures to provide supplementary information about emotional
responses.
HRV is especially suitable for studying social-emotional reg u-
lation, since it allows a much more detailed and continuous
determination of the regulatory characteristic of the ANS activ-
ity in response to psychophysiological stress. Although cortisol
change has been used to investigate within-person differences
in response to an acute stressor or momentary (state) emotion
(Dickerson & Kemeny, 2004; Adam, 2010), it is not easy to
collect saliva sample over and across the study time period.
Moreover, saliva cortisol levels take about 20 - 25 minutes post
stressor to peak, and take up to an hour to recover to pre-stress
baseline levels. Although some studies have linked momentary
negative mood states, such as anger, worry, and sadness to
acute increases in cortisol, most studies suggest that situations
that pose threats are the most consistent and powerful acute
activators of the HPA axis (Dickerson & Kemeny, 2004; Adam,
2012).
In the following sections, we will provide: 1) an outline of
the inheritability of and individual differences in cortisol; 2) a
brief summary of environmental influences on cortisol levels of
young children that are related to environmental controllability,
soc ial in ter act ion in the fa mily, and childcare upbringing; and 3)
discussions about the measurement and methodological chal-
lenges in salivary cortisol and HRV studies of young children
with a conclusion of possible directions for further research.
Inheritability and Individual Differences
Although psychologists agree that infant emotionality (pre-
sumed to be temperamental in origin) is rooted in biology, em-
pirical studies of the stability of infant emotionality have re-
vealed that sensitive and appropriately responsive parenting in
infancy is related to more optimal patterns of behavioral and
physiological reactivity and regulation. With environmental
experiences taken into account, findings in molecular genetics
research suggest that specific genes are related to infant emo-
tionality and later problems with depression, impulse control
problems, and externalizing/antisocial behaviors, especially when
paired with insensitive parenting or other adverse family envi-
ronments (Propper & Moore, 2006).
Variations in Inheritability
HPA axis polymorphisms have been linked to risk for de-
velopment of depression and posttraumatic stress disorder (Gil-
lespie, Phifer, Bradley, & Ressler, 2009) and individual differ-
ences in reactivity to laboratory-based stressors (Thode et al.,
2008). Moreover, the interactions between HPA axis polymer-
phisms and measures of early life adversity best predict stress
reactivity depression, and PTSD (Binder et al., 2008; Gillespie
et al., 2009; Tyrka et al., 2009). Besides differences in gene
sequence, additional genetic approaches have focused on epi-
genetic changes. Epigenetic changes are experience driven al-
terations to portions of the DNA that can serve to turn up or
turn down the expression of particular genes. Recent research
supports the possibility of experience-driven epigenetic pro-
gramming in humans.
Possible gene-environment interplay was found in estimating
the contributions of genes and environment to cortisol response
to stress in young children (Ouellet-Morin et al., 2008). Based
on 130 identical and 216 fraternal 19-month-old twins, their
study reveals that the genetic environmental bases of hormonal
response to stress depend on the context in which a child was
brought up. Patterns of differing genetic and environmental
contributions in cortisol reactivity to stress are found to be con-
tingent on familial adversity as high familial adversity may
20

C. PAWAN ET AL.
have a developmental effect that programs cortisol reactivity.
For children from a favorable family environment, genetics
account for 40% of the individual differences in cortisol re-
sponse to unfamiliar situations. However, for those growing up
in different family circumstances, the environment completely
overrides the genetic effect as if it had established a program-
ed hormonal conditioning to stress.
Individual Differences—Temperament as Predictors
of Emotion Reactivity and Regulation
Individual differences in emotion reactivity (i.e., response to
stimuli reflected in changes in the somatic, endocrine, and
autonomic nervous system) and emotion regulation (i.e., proc-
esses that adjust reactivity through appr oach, avoidance, or at t en-
tion mechanisms) have been considered genetic in origin and
stable over time and across contexts (Rothbart & Bates, 1998).
Temperamentally vulnerable children (e.g., fearful, anxious,
internalizing, and easily angered or frustrated) are more likely
to exhibit elevations in cortisol under conditions of less than
optimal care. A number of personal characteristics were shown
to influence cortisol activity level, including being a boy, more
socially fearful (Crockenberg, 2003), and emotionally negative
and having less self-control (Dettling, Parker, Lane, Sebanc, &
Gunnar, 2000). Responses of children to child care also display
individual variations, partly depending on how closely individ-
ual needs are met (Greenspan, 2003).
Traditionally, mid-morning and mid-afternoon levels of cor-
tisol have been used as an indicator of the effect of childcare.
Results of previous studies suggest that cortisol levels increase
or remain “flat” in young children across the day in school.
However, when an analysis of the childcare effect was done
with individual differences in temperament taken into account,
an interaction effect was found among groups of children with
different internalizing levels. Results revealed that the children
least internalized showed a significant decrease in cortisol lev-
els from morning to noon and after nap while cortisol levels of
the other groups across the day fit the pattern of the upward
curve (Li & Shen, 2008).
Moreover, individual differences and maturation of the cen-
tral nervous system (CNS) and autonomic nervous sy stem (ANS)
are considered as the foundation for emotional and behavioral
regulation. Individual differences in arousal and reactivity pre-
sent early in live have been suggested to be part of an individ-
ual’s temperament related to development of emotional experi-
ence and behavioral control (Fox & Calkins, 2003). Theories
focusing on the underlying physiological arousal and reactivity
of temperament highlight the maturation of the CNS and ANS
as the foundation for emotional and behavioral regulation (San-
tucci et. al., 2008). The ANS is considered primarily responsi-
ble for the physiological arousal related to emotional experi-
ences, resulting from input of both the excitatory sympathetic
nervous system (SNS) and inhibitory peripheral nervous system
(PNS). Two current theories regarding autonomic reflexivity
and emotional responding are Neurovisceral integration theory
(Thayer & Lane, 2000, 2009; Thayer & Ahs, 2012; Fredrikson,
Sollers III, & Wager, 2012) and Porges’ Polyvagal theory. The
PNS, usually measured by vagal control of the heart, has been
the primary focus of research on individual differences in tem-
perament-based responses to the environment (Calkins & Swing-
ler, 2012).
Environmental Sources of Influence on
Cortisol and HRV Levels
As mentioned earlier, individual differences in emotional re-
activity and emotion regulation exist with genetic bases. How-
ever, research results also support the instability of emotion,
suggesting possible influences of the environment (Pettit &
Bates, 1984; Wilson & Matheny, 1986). Although the contribu-
tion of environmental factors to emotion reactivity and regula-
tion is not well established, research has consistently found the
parent-infant relationship to be important to the development of
young children’s behavioral regulation, especially in early child-
hood (e.g., Crockenberg & Leerkes, 2004; Rosenblum, Mc-
Donough, Muzik, Miller, & Sameroff, 2002).
Social Interaction and Social-Emotional Regulation in
the Family
Recent research has studied how parenting influences the
underlying physiology and genetics of infant emotionality (for a
review, see Propper & Moore, 2006). Caregivers seem to play
important roles in regulating reactivity of the HPA system dur-
ing environment. In rodents, licking and grooming by the dam
and the delivery of milk into the gut maintain the adrenal hy-
poresponsive period, a period between postnatal days 4 and 14
when it is difficult to elevate glucocorticoid levels (Suchecki,
Rosenfeld, & Levine, 1993). In non-human primates, the pres-
ence of the mother serves to buffer activity of the HPA axis,
allowing the infant to behaviorally express distress to help elicit
maternal care without producing concomitant elevations in cor ti-
sol (Bayart, Hayashi, Faull, Barchas, & Levine, 1990). In hu-
mans, the attachment relationship between a caregiver and child
impacts cortisol reactivity (Gunnar, Larson, Hertsgaard, Harris,
& Brodersen, 1992; Lamb, 1998; Sims, Guilfoy le, & Parry, 2006;
Gunnar & Donzella, 2002). Cortisol levels of children with
secure relationships tend to return to basal levels more quickly
after the threat has been removed (Gunnar & White, 2001; Sims,
Guilfoyle, & Parry, 2006). Moreover, having secure relation-
ships impede the risk for increases in cortisol (Gunnar et al.,
1992; Gunnar & White, 2001). In general, empirical research
supports the theory that sensitive parenting in infancy is related
to more optimal patterns of physiological reactivity and regula-
tion.
Social support reduces stress levels in both animals and hu-
mans. In contrast, adverse experiences early in life may predis-
pose individuals to affective pathology through their effect on
the activity of the HPA system (Graham, Heim, Goodman,
Miller, & Nemeroff, 1999; Heim, Owen, Plotsky, & Nemeroff,
1997). A number of retrospective studies suggest that adults
who suffered emotional loss (e.g., loss of a parent), maladaptive
relationships with attachment figures, or maltreatment during
childhood exhibit heightened levels of corticotropin-releasing
hormone (CRH) and/or evidence of dysregulation of the HPA
axis (Gunnar & Donzella, 2002).
Family is a fundamental environment in which the relation-
ships determine the child’s development. Differing genetic and
environmental contributions according to familial adversity was
found in a recent study by Ouellet-Morin and colleagues (2008),
suggesting that high familial adversity may have a develop-
mental effect that programs cortisol reactivity. In conditions of
high familial adversity, both shared and unique environmental
factors (but not genetic factors) accounted for the variance in

C. PAWAN ET AL.
cortisol reactivity. The familial risk factors that affect the varia-
tions in heritability of cortisol reactivity include tobacco use
during pregnancy, low family income, low education level,
single parenthood, very early parenthood, low birth weight, and
maternal hostility toward the child (Ouellet-Morin et al., 2008).
However, human development is by nature a reciprocal interac-
tion. Therefore, the social relationships that are established
within the family environment may bear importance on the link
between cortisol response to stress and familial difficulties
during early development.
Environmental Controlla bility and Social
Competence
The development of emotionality is also a result of changes
of social-emotional experience in cognition and information
processing (Propper & Moore, 2006). Environmental controlla-
bility of individual experience can be an important influence on
behavior, personality, and response to live events (e.g., novelty).
Based on varied research and theory, it was proposed that ani-
mals and humans are naturally motivated to produce change in
their environment in order to build “competence” and that,
given the cumulative nature of “mastery” motivation, early child-
hood is the most fertile ground for its development (White,
1959; Roma, Champous, & Suomi, 2006).
Controllable or contingent stimulation in infancy facilitates
positive developmental outcomes including cognitive develop-
ment, exploration and learning motivation, and positive emo-
tional states (Gunnar, 1980a). Experiments based on the crea-
tion of an environment that develops “competence” (not just a
lack of “helplessness”) provide powerful endorsements of con-
trollability during infancy. Results showed that infants from the
master group were more exploratory in a novel environment
and less reactive during fear tests or stressful, novel situations
(Gunnar, 1980b; Mineka, Gunnar, & Champoux, 1986; Clarke-
Stewart, 1973; Joffe, Rawson, & Mulick, 1973; Roma, Cham-
pous, & Suomi, 2006). In a recent study, Gunnar and col-
leagues examined a group of preschoolers’ increases in salivary
cortisol from midmorning to midafternoon in full-time home-
based day care. Increases were found in the majority of children
(63%) at day care, with 40% classified as a stress response.
Observations at day care also revealed that intrusive, over-
controlling care was associated with the cortisol rise (Gunnar,
Kryzer, Van Ryzin, & Phillips, 2010).
Moreover, social competence is proposed to be a different
factor on coping than appetitive or inanimate controllability and
to have increasing salience in early childhood development
(Gunnar, 1980a; Roma, Champous, & Suomi, 2006). Accord-
ing to a study of rhesus monkeys, the coping advantages gained
by appetitive control were limited to the context congruent with
the individual mastery experiences and did not transfer to social
group situations (Roma, Champous, & Suomi, 2006). In studies
comparing human children’s HPA axis activity across home
and childcare settings, it was found that the amount and com-
plexity of play with peers and teacher-reported social fearful-
ness are related to the mid-afternoon increases in cortisol when
sampled at childcare, especially for toddlers (Watamura, Don-
zella, Alwin, & Gunnar, 2003). In future studies, aspects of
peer interaction (such as the amount of social control) could be
mapped to young children’s cortisol in daycare settings in order
to further understand the impact of social competence.
Childcare Environment
The HPA axis activation pattern is dependent on social con-
text. Based on the comparisons of children’s cortisol levels
across home and childcare settings, studies have shown mid-
morning to mid-afternoon increases in some children at child-
care but normal decreases at home during the same testing time
(Dettling, Gunnar, & Donzella, 1999; Dettling et al., 2000).
Moreover, response of children to childcare was found to be
dependent on: 1) the relationship between parent and child and
the child’s sense of psychological separation from parent (Jar-
vis & Creasey, 1991) and 2) the temperament characteristics
(such as internalizing) of the child (Li & Shen, 2008).
Children’s individual differences in physiological and be-
havioral reactions to stress also play a crucial role in so-
cial-emotional regulation in the childcare environment. Young
children who are either under- or over-reactive to stimuli are
especially vulnerable in childcare settings that do not tailor to
the child’s needs (Greenspan, 2003). Those who are highly
reactive to stimuli may become too distressed to elicit helpful
regulation processes (Coplan, Rubin, Fox, Calkins, & Stewart,
1994), and frequently distressed children may be more likely to
elicit negative responses from caregivers. Early physiological
reaction also contributes to later social competence at four years
of age (Calkins & Fox, 2002). Furthermore, children with less-
developed social skills were found to exhibit higher cortisol
levels and greater increases in cortisol in group care environ-
ments across the day period (Watamura, Donzella, Alwin, &
Gunnar, 2003).
Measurement and Methodological Challenges
Although pharmacological methods (e.g., dexamethaso ne sup-
pression test) may provide important information about the
activity of the HPA system, pharmacological tests have rarely
been used with young children. Most of the research on the
stress system in young children involves salivary measures of
cortisol since it is an easy and non-intrusive way of gathering
biological data and the release of cortisol can be reliably meas-
ured in saliva (Kirschbaum & Hellhammer, 1989). However,
the reliance on salivary cortisol measures also imposes limita-
tions related to measurement and methodological issues.
Developmen t al Changes, Dynamics of Cortisol
Secretion, and Timing of Sample Collection
Children’s cortisol levels are slightly lower than those of
adults and are characterized by great individual variability ( Si m s,
Guilfoyle, & Parry, 2006). A common phenomenon in past
studies signifying individual variations of cortisol secretion is
that standard deviations of salivary cortisol are large, close to or
even bigger than the mean values of salivary cortisol most of
the time (e.g., Davis, Donzella, Krueger, & Gunnar, 1999). In
addition to individual differences, a significant cortisol secre-
tion in young individuals is significantly affected from one day
to another. Mean cortisol levels vary significantly from one day
to another, even for a fixed sampling time, indicating signifi-
cant environmental influences on cortisol levels at similar time
points of different days (Li, Chiou, & Shen, 2007). Within sub-
ject cortisol levels differed significantly in the early morning,
early afternoon, and late afternoon cortisol data, and the sig-
nificance of the variation was related to the magnitude of the
correlation between cortisol and internalizing disposition. Mid-
afternoon cortisol levels showed the most significant day effect
22

C. PAWAN ET AL.
and the highest correlation with internalizing disposition.
Moreover, develop mental changes existi ng in cortisol r esponse
and basal concentrations need to be taken into account in study
designs. The hypo-responsive period of cortisol reactivity, a
period when the reactivity dampens, was found between 4 and
14 postnatal days for rodents and at about the first year for hu-
man infants (Suchecki, Rosenfeld, & Levine, 1993; Gunnar &
Donzella, 2002). However, little is known about 1) how envi-
ronmental influences, such as caregiver sensitivity and respon-
siveness, influence the maintenance of the hypo-responsive
period and 2) how other developmental changes affect cortisol
reactivity and basal concentrations of cortisol during infancy or
over the course of early development.
Novelty has been believed to increase cortisol levels. How-
ever, new activities that engage attention may produce a de-
crease instead of an increase in cortisol levels for young chil-
dren. New activities or novel events such as a car trip (Larson
et al., 1991), swimming lessons (Hertsgaard, Gunnar, Larson,
Brodersen, & Lehman, 1992), taking part in a play group (Leg-
endre & Trudel, 1996), and attending childcare settings (Det-
tling et al., 1999) were shown to decrease cortisol in infants and
preschoolers (compared to their ho me baseline), especially wh en
the exposure to novelty occurred in the mother’s presence and
the novelty elicited generally positive affect. Preschool-aged
children attending a half-day nursery program, regardless of
morning or afternoon sessions, show lower cortisol levels as
compared to the home baselines (Gunnar et al., 1997). However,
the effects of novelty described above were no longer seen in
children five years and older (Dettling et al., 1999; Gunnar &
Donzella, 2002).
Play with peers is fun, but the difficulty in learning to make
friends and play nicely with peers is challenging varies with age.
For most young children, social competence will improve with
age over the early childhood years. However, the development
of skilled social interaction with peers is more challenging for
children with internalizing disposition and may be related to
cortisol increase over the day in childcare settings (Gunnar &
Donzella, 2002; Li & Shen, 2007). Young children between 21
and 40 months of age, when children become highly motivated
to make friends and play with peers, were found to show
greater increase in cortisol over the day than children of other
ages. Gunnar and Donzella (2002) suggest that the rise in cor-
tisol over the childcare day emerges at about the age when peer
relations become a focus of young children in group-care set-
tings and that the increase in cortisol diminishes with the in-
crease in social competence. However, the hypothesized links
need to be examined prospectively in order to better understand
the long-term consequences of early experiences.
Thus, in order to detect the interplay of environmental influ-
ences and individual characteristics and to avoid under-repre-
sentation of the correlations between personality traits and cor-
tisol responses, one could use a data aggregation method and
select the optimal sampling time. However, as there are clear
day effects, in order to better understand the dynamics of corti-
sol levels, further research must observe and recode the activi-
ties of the individual child, peers, and caregivers/teachers in the
classroom to investigate possible sources of variation and ex-
plore situational effects with HRV data.
HRV Data as In dicators of Temperame nt and
Emotion Re gulation
Cardiac vagal tone has been considered as a psychophysi-
ological marker of emotion regulation and arousal (Porges,
1995). Over the past decades, HRV has been used increasingly
to analyze changes in sympathovagal balance related to indi-
vidual characteristics such as temperament and coping strate-
gies. HRV has been successfully used as a measure of auto-
nomic regulation of cardiac activity in human and animal stud-
ies to assess stress and well-being under various conditions and
to characterize and understand individual traits such as tem-
perament and coping strategies, in both human and animal
studies (von Borell et al., 2007). von Borell and colleagues (von
Borell et al., 2007) claim that the same psychophysiological
principles can be applied to humans and non-human mammals
based on a thorough review of related studies.
Baseline resting levels of vagal tone has been found to be re-
lated to individual differences in reactivity and soothability of
young children (e.g., Calkins, 19 97; Calkins & Fox, 2002; St i f t e r
& Fox, 1990). Moreover, low resting vagal tone was found to
be generally related to negative affectivity (Beauchaine, 2001),
while high vagal tone was found to be associated with approach
to strangers, high activity level, lower levels of aggression, and
regulated distress in frustrating situations for toddlers (Calkins
& Dedmon, 2000; Porges, Doussard-Roosevelt, Portales, &
Greenspan, 1996; Stifter & Jain, 1996) and greater empathy,
social competence, and subjective feelings of sympathy, and
sociability and emotion regulation of young boys (Eisenberg,
Fabes, Murphy, Maszk, Smith, & Karbon, 1995; Fabes, Eisenberg,
& Eisenbud, 1993; Fabes, Eisenberg, Karbon, Troyer, & S witzer,
1994). On the other hand, suppressed vagal tone during a chal-
lenging task was claimed to be related to regulation of attention
and behavior and may facilitate orientation to stimuli (Calkins,
1997; Porges, Doussard-Roosevelt, & Maiti, 1994).
Study the Complex Oscillations of HRV Data
Healthy cardiac function is characterized by irregular time
intervals between consecutive heart beats. The rhythmic oscil-
lation of the regulatory components of cardiac activity that
function to orchestrate responses to challenges and to maintain
cardiovascular homeostasis contributes to the complex oscilla-
tions of HRV data. An oscillatory curve can be produced when
consecutive IBIs are plotted on a time scale. The “mixed oscil-
lation” of this curve results from the rhythmic pulses of the
different regulatory components, where rhythmic activities
originating from the PNS exhibit higher frequency than those of
the SNS. In order to analyze the complex oscillations of HRV,
using data from at least 5-min of consecutive IBIs is recom-
mended (von Borell, et al., 2007).
According to Borell and colleagues (Borell et al., 2007), re-
cordings of IBIs should contain less than 5% of artefacts before
editing and subsequent manual editing of the data should be
done to a very high standard. They also identified the following
areas that warrant further study in order to improve methodol-
ogy and to enhance our understanding of HRV and underlying
sympathovagal mechanisms in relation to stress and emotional
regulation:
1) Improve ease of analysis by means of automatic elimina-
tion of artefacts.
2) Measure possible confounding effect, such as diurnal
variation and effects age, sex, sleep, metabolic state and other
factors on HRV, then find ways to eliminate or minimize the
confounding effects (e.g., standardize the data according to age,
sex and time of the day).

C. PAWAN ET AL.
3) Study possible age/temperament specific ranges of varia-
tion for HRV in the populations in order to estimate subject
numbers needed for studies comparing HRV in response to
intrinsic and environmental/social factors.
4) A within-subject change in HRV, recorded before and af-
ter a treatment is applied, is more meaningful than between
subject/group comparisons.
Conclusion
The moderating effect of individual differences must be con-
sidered in future research of environment influences on cortisol
levels of young children. Moreover, considering the dynamics
and individual differences in cortisol secretion, it is important
to detect the interplay of environmental influences and individ-
ual characteristics.
When studying the environmental influences, consideration
of the multi-faceted constructs related to early development is
necessary. Possible mediating effects of environmental factors
may include interaction and social-emotional regulation in the
family, environmental controllability and social competence,
and the childcare environment including social interaction with
peers. Prospective examination is needed on the long-term out-
comes of various individual characteristics and environmental
influences in early experience that are related to reactivity dif-
ferences and atypical patterns of cortisol, both hypercortisolism
and hypocortisolism.
To avoid under-representation of the links between personal
characteristics and cortisol responses, one may try a data ag-
gregation method and determine the optimal sampling time of
salivary cortisol. Considering the dynamics of cortisol levels,
further research must observe and recode the activities of the
individual child, peers, and caregivers in the classroom in order
to investigate possible sources of variation and to explore situ-
ational effec ts .
Future research should provide a more complete picture of
the temporal course of emotional reactivity and regulation.
Measuring emotional regulation ability by using physiological
indicators, one should take into account the baseline pattern, the
reactive response, and the tendency of reco very to bas eline.
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