Both males and females can get sleepy after sex. They can't help it. It's the chemicals released after orgasm.
There are set of specific reasons as to why this tiredness occurs. In most cases it is a combination of exerting a lot of physical energy during sex as well as the chemicals that are released when the act is complete and orgasm is achieved.
"Then there is the biochemistry of the orgasm itself. Research shows that during ejaculation, men release a cocktail of brain chemicals, including norepinephrine, serotonin, oxytocin, vasopressin, nitric oxide (NO), and thehormone prolactin. The release of prolactin is linked to the feeling of sexual satisfaction, and it also mediates the “recovery time” that men are well aware of—the time a guy must wait before “giving it another go.” Studies have also shown that men deficient in prolactin has faster recovery times.
"Prolactin levels are naturally higher during sleep, and animals injected with the chemical become tired immediately. This suggests a strong link between prolactin and sleep, so it’s likely that the hormone’s release during orgasm causes men to feel sleepy.
(Side note: prolactin also explains why men are sleepier after intercourse than after masturbation. For unknown reasons, intercourse orgasms release four times more prolactin than masturbatory orgasms, according to a recent study.)
"Oxytocin and vasopressin, two other chemicals released during orgasm, are also associated with sleep. Their release frequently accompanies that of melatonin, the primary hormone that regulates our body clocks. Oxytocin is also thought to reduce stress levels, which again could lead to relaxation and sleepiness."
In human sexuality, the refractory period is usually the recovery phase after orgasm during which it is physiologically impossible for a man to have additional orgasms. Though it is generally reported that women do not experience a refractory period and thus can experience an additional orgasm, or multiple orgasms, soon after the first,some sources state that both men and women experience a refractory period because women may also experience a period after orgasm in which further sexual stimulation does not produce excitement. C: Sumit Bisht
Why do men get so tired after sex?
Why Do Men Feel Sleepy After Sex? Prolactin, Oxytocin, Vasopressin, et al.