It's pretty common to feel drained after masturbating, but this is mostly a direct result of an orgasm. An orgasm helps release endorphins into your bloodstream which causes pleasure and relaxation, thus making you feel sleepy or lethargic. There's also a matter of perception, studies show that the relaxing or tiring effect of masturbation may be caused by people believing that it will make them exhausted. That is to say, if you believe doing this will tire you out, chances are it will.