One of the most obvious reasons you feel sleepy after sex is that it can be physically exhausting. During sex, your blood pressure and blood flow goes up just like during an exercise, and the body releases endorphins. Endorphins are chemicals that your body naturally produces to relieve stress.
During sex, the brain releases a hormone called oxytocin which is also called the “love hormone” because it heightens the feeling of affection, arousal and attachment. But when you orgasm, the hormone wears off, and this can make you feel tired, relaxed and sleepy.
When the body releases oxytocin, it also releases melatonin, which is the main hormone that regulates sleep.
Other hormones released in both men and women during sex include: vasopressin, prolactin, serotonin, nitric oxide, and endorphins. Some of these hormones are released when you orgasm, so whether or not you orgasm can make a difference in how sleepy you are.
Note that some men and women may react differently to some of these chemicals. If someone is very stressed and they have sex, they are likely to get more tired after they ejaculate or have an orgasm.