Level of evidence:
Frequently asked questions about sleep and its hormones
How many sleep cycles do you have per night?
Usually 4 to 6 cycles of about 90 minutes. Deep sleep dominates the first cycles, and REM sleep (the dreaming kind) gets longer toward morning. Waking briefly between two cycles is normal (Patel, StatPearls).
When during the night is growth hormone released?
Mostly early in the night, with the first deep sleep: it is the biggest peak of the day in adults (Van Cauter, 2000).
Why do you wake up around 3 or 4 a.m.?
Often at the end of a cycle, when sleep becomes lighter. Cortisol also starts to rise again in the second half of the night, to prepare for waking (Van Cauter, 2000). A brief awakening is normal; what is bothersome is not falling back asleep.
Does lack of sleep lower testosterone?
Yes: one week of 5 hours of sleep per night lowers testosterone by 10 to 15% in young men (Leproult, 2011).