jeez... Ignore the other guy. Masturbating is actually good for your body. I wont go into too many details as that is not what the question is asking. Basically, when you masturbate and ejaculate, you're body releases hormones into your system that calms you, make you relax, and makes you sleepy, which is why people often sleep after having sex. Try masturbating before you go to sleep, and I guarantee you'll sleep a lot better than normal. Also try masturbating during stressful times.

masturbating is bad for your health so regular sex wont make you tired like that

anonymous said it perfectly (hormones, etc.)... Think of it as like climbing a tall mountain - on the way up (arousal) you're going higher and higher with sexual tension (requiring lots of energy, too). Then you cross over the peak (orgasm) and head down a very steep slope on the other side, where your body is programmed to rest and recover.

Thank God for these wonderful mountains - try to enjoy all parts of the trip, up - over the peak - and back down!