Why man fall asleep after fucking?

Some girls say ot think that it is sleeping pills or we
can't fuck anymore because we are empty or it needs to
re-charge again.
Some also think we fall asleep after sex
because it was bedtime. They mean usually they're fucking at
night anyway so it made sense to them, right?

Well
according to the medical community, chemical shit happens in
mens bodies after an orgasm.
  
I found several websites
with similar material, 
After orgasm, both men and women
release the chemicals oxytocin, prolactin, gamma amino
butyric acid (GABA), and endorphins. Each of these
contributes to that roll-over-and-snore feeling. They seem
to be secreted in equal amounts in men and women, but we all
know who orgasms more frequently. 
The hormone oxytocin is
known to have several effects including establishment of
maternal behavior, stimulation of uterine smooth muscle
contraction at birth, and stimulation of milk ejection (milk
letdown). It is also referred to as 'the cuddling hormone'
since it tends to elicit the need to be close and bond but
not in a sexual way. In one study, oxytocin was shown to
inhibit male sexual behavior in prairie voles. Maybe it's
the oxytocin that makes us feel satiated and rested after a
good romp. 
Prolactin is another player in the sex/sleep
conundrum. It is produced in the pituitary gland and its
best known function is the stimulation of milk production.
Prolactin is believed to relieve sexual arousal after orgasm
and take your mind off sex. Levels of prolactin rise during
sleep and some patients with prolactin-secreting tumors
report frequent sleepiness. So prolactin seems like it may
be a culprit too. 
Gamma amino butyric acid (GABA) and
endorphins also both have a calming effect and may make you
pass out post-sex. 
The tantric answer is that the male
orgasm releases outward energy or jing, while the female
orgasm is an inward explosion. 
Last but not least, we have
Grandma's reasoning. It is thought that exertion during sex
and after climax depletes the muscles of energy-producing
glycogen. This leaves men feeling sleepy. Since men have
more muscle mass than women, men become more tired after
sex. Also (believe it or not!) women don't always have an
orgasm, and that keeps them from producing all those other
hormones we just talked about