Determinants of cortisol awakening responses to naps and nighttime sleep.

The cortisol awakening response (CAR) is a phenomenon describing the sharp increase in basal cortisol levels shortly after waking from sleep. While extensively studied, little is known about the role of sleep architecture contributing to CAR. Furthermore, the potential for CAR after a shorter bout of sleep--a nap--has not been directly investigated. The current studies thus aimed at assessed sleep duration, time of day, and sleep architecture as potential determinants of the cortisol awakening response. Saliva samples were collected during the first hour (0, 30, 45, 60 min) following several EEG-monitored laboratory sleep conditions. Those included afternoon naps wherein 17 participants (4 men; ages 18-26) napped for 50 min and 24 participants (11 men; ages 18-24) napped for 90 min. Furthermore, 20 participants (10 men; ages 18-35) visited the lab twice and in addition to staying overnight, napped 90 min in the morning either under placebo conditions or pharmacologically-manipulated sleep conditions (5mg Zolpidem). Cortisol increases were observed in response to each sleep condition except to 50-min afternoon naps. Furthermore, CARs were predicted by Stage 2 sleep when following nighttime sleep (r=.46, p=.04) and by Stage 1 sleep when following placebo morning naps (r=.54, p=.01). The current study established cortisol awakening responses to naps and implicates sleep duration and architecture in the generation of CAR to both napping and nighttime sleep. Assessing CAR in conjunction with the specific type of sleep may thus contribute to our understanding of mechanisms underlying positive and negative health effects of napping.