Recurrent short sleep, chronic insomnia symptoms and salivary cortisol: A 10-year follow-up in the Whitehall II study

Although an association between both sleep duration and disturbance with salivary cortisol has been suggested, little is known about the long term effects of poor quality sleep on diurnal cortisol rhythm. The aim of this study was to examine the association of poor quality sleep, categorised as recurrent short sleep duration and chronic insomnia symptoms, with the diurnal release of cortisol. We examined this in 3314 participants from an occupational cohort, originally recruited in 1985-1989. Salivary cortisol was measured in 2007-2009 and six saliva samples were collected: (1) waking, (2) waking + 0.5 h, (3) +2.5 h, (4) +8 h, (5) +12 h and (6) bedtime, for assessment of the cortisol awakening response and the diurnal slope in cortisol secretion. Participants with the first saliva sample collected within 15 min of waking and not on steroid medication were examined. Short sleep duration (≤5 h) and insomnia symptoms (Jenkins scale, highest quartile) were measured in 1997-1999, 2003-2004 and 2007-2009. Recurrent short sleep was associated with a flatter diurnal cortisol pattern. A steeper morning rise in cortisol was observed among those reporting chronic insomnia symptoms at three time points and among those reporting short sleep twice, compared to those who never reported sleep problems. Participants reporting short sleep on three occasions had higher levels of cortisol later in the day, compared to those never reporting short sleep, indicated by a positive interaction with hours since waking (β = 0.02 (95% CI: 0.01, 0.03)). We conclude that recurrent sleep problems are associated with adverse salivary cortisol patterns throughout the day.