COMPARATIVE EFFECT OF NIGHT AND DAYTIME SLEEP ON THE 24-HOUR CORTISOLSECRETORY PROFILE

To determine whether cortisol secretion interacts with daytime sleep i n a similar manner to that reported for night sleep, 14 healthy young men were studied during two 24-hour cycles. During one cycle they slep t during the night, during the other the sleep period was delayed by 8 hours. Secretory rates were calculated by a deconvolution procedure f rom plasma cortisol, measured at 10-minute intervals. The amount of co rtisol secreted during night sleep was lower than during the correspon ding period of sleep deprivation (12.7 +/- 1.1 vs. 16.3 +/- 1.6 mg; p < 0.05), but daytime sleep beginning at the habitual time of morning a wakening failed to inhibit cortisol secretion significantly. There was no difference between the amount of cortisol secreted from 0700 to 15 00 hours in sleeping subjects and in subjects who were awake during th e same period of time (24.2 +/- 1.5 vs. 22.5 +/- 1.4 mg). Even if the comparison between sleeping and waking subjects was restricted to the period 0700-1100 hours or 0700-0900 hours, no significant difference w as found. Neither secretory pulse amplitude nor frequency differed sig nificantly in either period. However, detailed analysis of the secreto ry rates in daysleepers demonstrated a transient decrease in cortisol secretion at about the time of sleep onset, which began 10 minutes bef ore and lasted 20 minutes after falling asleep. Spontaneous or provoke d awakenings had a determining influence on the secretory profiles. Te n to 20 minutes after awakening from either night or day sleep cortiso l secretion increased significantly. The main secretory episode in the early morning, which reflects the interaction between circadian proce sses and awakenings, did not differ in its timing between night and da y sleepers, providing evidence of the strength of the circadian rhythm . These findings demonstrate that daytime sleep beginning at the circa dian cortisol acrophase, when compared to wakefulness, did not induce a significant decrease in the amount of cortisol secreted during the s ubsequent 2- or 4-hour period, despite a transient decrease in secreti on around the time of sleep onset.