Relationship functioning and home and work demands predict individual differences in diurnal cortisol patterns in women
Introduction
Recent evidence has shown relationships to be both powerful instigators (Flinn and England, 1995) and powerful buffers (Nachmias et al., 1996) of acute stress hormone reactivity in humans. There has been less research, however, on the role of relationships in understanding the basal organization of stress hormone activity. The current study examines the associations between relationship functioning and the organization of cortisol levels across the waking day in a healthy sample of adult women. In addition, it examines the associations between women's home and paid work demands and the organization of their diurnal cortisol patterns.
Cortisol is the main product of the hypothalamic–pituitary–adrenal (HPA) axis, one of the primary stress-responsive systems in humans. Cortisol may be non-invasively and reliably measured in small samples of human saliva (Kirschbaum and Hellhammer, 1989, Kirschbaum and Hellhammer, 1994), making it an attractive method for repeated measurement and for collection in naturalistic settings. In adults, cortisol levels typically have a strong diurnal pattern: they are highest in the morning soon after waking, drop rapidly in the first few hours after waking, then continue to drop more slowly across the day, reaching a nadir in the first few hours of sleep (Weitzman et al., 1971, Lacerda et al., 1973, Van Cauter, 1990, Schmidt-Reinwald et al., 1999). This diurnal pattern is established early in life, first emerging at about three months of age (Price et al., 1983).
While considerable research exists on factors predicting cortisol reactivity to stressors in adults, very little research is available on factors predicting individual differences in diurnal cortisol rhythms, particularly in normal adults in their naturalistic settings. When investigators have examined cortisol activity in nonclinical populations, they have tended to control for time of day by gathering data at a constant time of day rather than considering it a variable of interest. The way in which cortisol levels change across the day is however a potentially important indicator of the functioning of the HPA axis (Smyth et al., 1998). In general, because a strong diurnal rhythm in cortisol (with high morning levels, low evening levels, and a strong negative slope) is the normative or expected pattern, researchers have assumed an extremely weak, inconsistent or absent diurnal cortisol rhythm to be a sign of HPA dysregulation (Caplan et al., 1979). Whether or not more subtle individual differences in diurnal cortisol patterns have any psychological, physiological or clinical significance, however, remains an open question.
In clinical populations, evidence of a flattening of the diurnal cortisol rhythm (usually indicated by a smaller drop in cortisol from morning to afternoon or evening compared to a control group) has been reported for a variety of groups, including depressed adults (Carrol et al., 1976), children with a history of maltreatment and current symptoms of depression (Kauffman, 1991, Hart et al., 1996) and children reared in institutional settings (Carlson and Earls, 1997). A flattening of the diurnal cortisol rhythm has also been found in several physical disorders, including fibromyalgia (Crofford et al., 1994, McCain and Tilbe, 1989), chronic fatigue syndrome (MacHale et al., 1998) and severe rheumatoid arthritis (Neeck et al., 1990).
Very few studies have systematically examined factors predicting diurnal cortisol patterning in normal adults. An early study of this nature found lower morning cortisol levels and less of a drop to afternoon levels among individuals experiencing high levels of work stress (Caplan et al., 1979). A recent study on the relations between employment and diurnal cortisol activity reported unemployed subjects to have higher morning levels and lower evening levels than employed subjects (Ockenfels et al., 1995). In another analysis of these data, individuals who were judged to show flat diurnal cortisol cycles were no different from individuals with normal or inconsistent cycles on a wide variety of demographic and psychological variables (Smyth et al., 1997). Another recent study also found no significant associations between personality variables and change in cortisol levels across the day (Schommer et al., 1999).
Thus while there is some evidence that current job strain may be associated with the organization of cortisol levels across the day, the available evidence does not point to associations between more stable aspects of psychological functioning and diurnal cortisol patterns. Yet prior studies have not focused on one aspect of psychological functioning that may be particularly important for stress system functioning: the quality of an individual's functioning in relationships. Interpersonal relationship style or functioning may be important because stress hormone activity is dependent not only on the nature of the stressors faced in the current environment, but also on one's assessment of available coping resources (Kirschbaum and Hellhammer, 1989). Relationships are a central resource for coping with life's challenges from infancy through to adulthood (Sroufe, 1996, Cohen and Wills, 1985). Importantly, it is not just the presence of relationships, but the quality of relationships that matters for understanding stress hormone activity (Spangler and Grossmann, 1993, Hertsgaard et al., 1995, Gunnar, 1999). For example, temperamentally fearful toddlers with secure attachment relationships with their mothers have been shown to have reduced cortisol reactivity to a novel stimulus compared to fearful infants with insecure attachments (Nachmias et al., 1996). In another study, men (but not women) who received social support from their romantic partners prior to a social stress paradigm had reduced cortisol reactivity to the stressor compared to those who received social support from a stranger or no support (Kirschbaum et al., 1995). In addition to serving as a coping resource or buffer against stress reactivity, relationships can be a significant source of emotional stress and a powerful initiator of physiological stress reactivity. Flinn and England (1995) found that traumatic family events (such as conflict, punishment, shaming, quarreling and fighting) predicted elevations in cortisol in naturalistic settings more strongly than any of the other variables measured.
What role might relationships play in the diurnal organization of cortisol activity? Several of the studies mentioned previously suggested associations between early and severe relationship stress (maltreatment, institutional rearing) and the diurnal pattern of cortisol activity (Hart et al., 1996, Kauffman, 1991, Carlson and Earls, 1997). No prior studies have focused on the links between more subtle variations in relationship experience and the organization of cortisol activity across the day in normal adults. In a primarily middle-class sample of mothers of two-year old children, the current study examines the associations between measures of relationship functioning and the patterning of mothers' cortisol levels across their waking day. Because several prior studies have found effects of employment on diurnal cortisol patterning, aspects of mothers' employment and the demands placed on mothers in their home environments are assessed. Finally, a variety of medical and demographic control variables are examined.
In addition to examining associations between relationship functioning, work and home demands, and diurnal cortisol activity, this study adds to the literature by using Hierarchical Linear Modeling (HLM) growth curve analysis techniques (Bryk and Raudenbush, 1992) to better model and predict individual differences in the patterning of cortisol across the day in a normal sample of adult women.
Section snippets
Participants
Participants were 70 mothers of toddlers who were recruited from a larger study of factors contributing to parenting stress in mothers of young children. They were predominantly Caucasian (98%), married (96%), middle-class (mean income $54,000) and college-educated (mean years of education 16.4). Most were employed either part-time (37%) or full-time (29%); the remainder (34%) were full-time homemakers. The mean age of mothers was 34 years (range 24 to 42), the mean age of their toddler was 25
Stability in cortisol levels from Day 1 to Day 2
Participants' average cortisol levels (mean of all available samples each day) from Day 1 to Day 2 were strongly correlated (r=0.65; P<0.001). Correlations between individual morning, afternoon and evening samples (controlling for exact time of sampling within each time period) were more variable, as would be expected given that individual values will reflect trait level cortisol as well as fluctuations due to immediate stressors and/or ultradian spikes (Kirschbaum et al., 1990). The
Discussion
This study finds systematic associations between contextual and psychological variables and individual differences in the organization of cortisol levels across the normal waking day in a community sample of adult women. First it was shown that participants' cortisol levels were strongly predicted by time of day and their daytime cortisol curves could be modeled by parameters representing morning cortisol values, the rate of decline of cortisol across the day and average cortisol levels. It was
Acknowledgements
The authors would like to thank the women who generously volunteered their time to participate in this research. We would like to thank Laurie Brodersen and the graduate and undergraduate students who assisted with data collection for this study, and the Endocrine Laboratory at the University of Minnesota for conducting the salivary cortisol assays. We are also grateful to the anonymous reviewers who provided valuable feedback on this manuscript. This research was supported by doctoral and
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