Prolonged perceived stress predicts fingernail cortisol increase: A longitudinal study

Cortisol levels in fingernail samples are known to represent cumulative hormone levels. However, findings on the association between fingernail cortisol and perceived stress have been inconsistent. Since the effect of confounding factors cannot be excluded in cross-sectional studies, this study longitudinally examined the effects of perceived stress and negative life events on changes in fingernail cortisol levels over a period of six months. In total, 113 university students rated their perceived stress and negative life events twice, fingernail samples were collected every 15 days during a six-month period. Furthermore, fingernail cortisol levels were quantified according to each stress and negative life event assessment. We conducted longitudinal multilevel analyses to estimate the effects of perceived stress and negative life events in changes within an individual (within-person change) as well as individual differences of them over the study period (between-person level) on changes in fingernail cortisol levels. Within-person increase in perceived stress and negative life events did not significantly predict within-person increase in fingernail cortisol levels. In the between-person level, higher average levels of negative life events did not significantly predict the rate of increase in fingernail cortisol levels than those with lower levels. However, individuals with higher average levels of perceived stress showed a steeper slope of time in fingernail cortisol levels. These findings suggest that prolonged high levels of perceived stress may be associated with fingernail cortisol levels.