Because the physiological result of orgasm is exhaustion. It’s normal to sleep after orgasm. The reasons are multiple, including release of hormones known to cause sleep, and mental relaxation that calms the mind.

From the LiveWire article:

Then there is the biochemistry of the orgasm itself. Research shows that during ejaculation, men release a cocktail of brain chemicals, including norepinephrine, serotonin, oxytocin, vasopressin, nitric oxide (NO), and the hormone prolactin. The release of prolactin is linked to the feeling of sexual satisfaction, and it also mediates the “recovery time” that men are well aware of—the time a guy must wait before “giving it another go.” Studies have also shown that men deficient in prolactin have faster recovery times.

Prolactin levels are naturally higher during sleep, and animals injected with the chemical become tired immediately. This suggests a strong link between prolactin and sleep, so it’s likely that the hormone’s release during orgasm causes men to feel sleepy.

(Side note: prolactin also explains why men are sleepier after intercourse than after masturbation. For unknown reasons, intercourse orgasms release four times more prolactin than masturbatory orgasms, according to a recent study.)

Oxytocin and vasopressin, two other chemicals released during orgasm, are also associated with sleep. Their release frequently accompanies that of melatonin, the primary hormone that regulates our body clocks. Oxytocin is also thought to reduce stress levels, which again could lead to relaxation and sleepiness.

You’re not weak, you’re sleepy and relaxed.

That day I visited a medical practioners.
Chinese one. Tabib.
He say don't fap too much.
Becuz sperm is nutrients. And when u fap u lose yang energy.
So it will cause alot major issue
Hair loss
Skin damage
Panda eye
Sleepy all the time
Loss of social skills
Etc.

So after listening to him I already stop fapping for a week.
I do feel increase in energy level and feel my complexion is improved.
But I feel horny all the time.

I am considering to stop fapping entirely.
What say u?

i only know my face got acne problem if i fap too often

until today i got no solution 

You keep fapping where you get energy entertain your gf?

fap until on the verge of ejaculation then stop la. let the sperm go back to the balls

It's okay, if you lose your hair, can come see me.

Since he said sperm is nutrients, you should drink your sperm after every fapping session. Just recover what you loss.