Prolactin is another player in the s*x/sleep saga. It is produced in the pituitary gland, and its best-known function is the stimulation of milk production. Prolactin is believed to relieve s*xual arousal after climax and take your mind off s*x. Levels of prolactin rise during sleep, and some patients with prolactin-secreting tumors report sleepiness. So prolactin seems like it may be a culprit.
GABA and endorphins also both have a calming effect and may make you pass out after s*x. So why does the postcoital snooze seem to be so much more a man thing?
This may come as quite a shock to some of you, but men are known to release in various semicomatose states, such as during prostate exams and while thumbing through dog-eared issues of National Geographic magazine, women—believe it or not!—don’t always have orgasms during s*x, and that keeps them from producing all those other hormones.