Sleep and Salivary Cortisol

The aim of the present chapter was to analyze whether measures of cortisol in saliva were associated with measures of sleep and to explore if divergent results were related to underlying differences in theoretic assumptions and methods. Measures of sleep quality included sleep duration, overall sleep quality, difficulty falling asleep, disturbed sleep, and sleep deprivation. Twenty-three papers were found to fulfil the inclusion criteria. Cortisol measures were grouped into single time points at different times during the day, deviations at different time periods during the day, reactivity and recovery after a standardized laboratory test, area under the curve and response to dexamethasone test. A large proportion of the studies included showed nonsignificant findings, which, in several cases, may be a result of low power. The most consistent results were a positive association between sleep duration and single measures of salivary cortisol at awakening, which was observed in 3 studies. In these studies, sleep duration was also associated with low evening cortisol levels, steep diurnal deviation of cortisol and/or high area under the curve. Together these findings suggest that longer sleep duration is related to a more dynamic cortisol secretion. Two of the 6 studies on disturbed or restless sleep showed relations to flat diurnal deviation and low laboratory stress test reactivity. This to some extent corroborates the findings on sleep duration. However, the many nonsignificant findings as well as the theoretical and methodological differences (e.g., heterogeneity in measures) complicate comparisons. Conflicting results may be at least partially due to differences in methods and underlying assumptions.

Keywords: Salivary cortisol, sleep, sleep quality, sleep duration, sleep deprivation, difficulty falling asleep, single time point measures, deviations measures, area under the curve, dexamethasone.