Hormones elevated early in sleep and elevated late in sleep
Recall that SWS predominates during the first third of sleep and REM predominates during the last half of sleep. GH and prolactin (PRL) are entrained to SWS. The increase in GH secretion during sleep is well known. Regardless of age and gender, most PRL secretion also occurs when the individual is asleep, with nocturnal maximum PRL levels twice that of daytime levels. The nighttime GH and PRL surges are associated with the first period of SWS. In fact, the GH surge immediately after sleep onset is the largest of the 24-hour period for both genders, although in females the surge is less than in males. Females have two evening GH surges; the first is before sleep onset, and a second occurs with SWS. Males have few daytime GH pulses compared with females. The surges for PRL and GH are lost during a night of total sleep deprivation and return during daytime recovery sleep. It is the onset of sleep and not the time of day that primarily triggers GH and PRL release, please note PRL also has the stronger circadian trigger.
The hormones that increase later in sleep are testosterone and cortisol. Testosterone rises just after midnight and cortisol begins its rise at 2 am, peaking at 6 to 9 am. The timing and amount of REM sleep are related to the late-sleep rise of these two hormones in men. But the 24-hour rhythm for both testosterone and cortisol is primarily controlled by circadian rhythmicity (process-C) and not sleep–wake homeostasis (process-S). Before we leave this subject, it is well known that many drugs increase serum prolactin levels (e.g., narcotics, antiemetics, antipsychotics). Added to this list are the benzodiazepine sedatives and imidazopiridine hypnotics, such as triazolam and zolpidem. Taking either at bedtime is associated with nocturnal, but not daytime, marked increases in serum prolactin.