Now, Why do we get sleepy after having sex? It's very simple. The man's body makes a huge effort to achieve orgasm and all that energy expenditure, when relaxing, is what causes sleep.
In addition, during ejaculation, the man's brain secretes various hormones, many of them related to sexual satisfaction; But one, especially prolactin, will make a man sleepy after intercourse.
We already have the perfect excuse to give your wife if you fall asleep after having sex!