Why do men fall asleep right after sex, but not women?

Great question, that gets asked quite a lot, because this behavior seems to be a common occurrence – although there really isn’t any evidence to back up this belief.  I don’t know of any formal study, but there are a few theories.  The act of erection and ejaculation redirects blood flow from the brain, reducing oxygen, and may cause a feeling of drowsiness.  There is also muscle constriction in the pelvis and groin area.  Prolonged muscle tension will cause muscle fatigue and the need for relaxation. When this part of the body becomes ore relaxed, the rest of the body may become more relaxed as well. The actual physical activity involved, particularly for the man in the traditional missionary position, may also be a factor.  The act of ejaculation is euphoric, which may lead to a feeling of relaxation.