Why does sex make me sleepy?

I’ve heard of men who fall asleep after sex, but I am female. When I say ‘sex’, I am referring to clitoral stimulation. It is true that I have fallen asleep when it has been very late at night, but I need not be exhausted to go into a half-sleep mode. I have also fallen asleep while stimulating another’s clitoris. Are there chemical reasons for such sleepiness?