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When we have a sex with partner, we feel so tired that we don't have any power to do anything. Then it would be disadvantage to the animals, of course humans too, for they are exposed to danger. For example, the predator's atack. But you know, we cannot help having a sex. Then what is the advantage of being tired after having a sex?

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    $\begingroup$ Why do you think there should be an advantage to everything that exists? And not everyone feels exhausted after sex. $\endgroup$ – WYSIWYG Jul 19 '15 at 14:55
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    $\begingroup$ The reason why we feel relaxed / sleepy after having sex is probably the release of endorphins, i.e. morphine-like chemicals. This is part of what makes the experience pleasurable. In a sense, we are rewarded with endorphins for having sex. Probably, a reason why we evolved this way is we would not mate as much if there was no reward for the behavior, hence the trait favors reproduction. We must sleep anyway. It is more advantageous to do so after having sex (or having eaten for that mater) than to sleep at other times, which would possibly make one miss an opportunity to mate or feed. $\endgroup$ – Erwan Legrand Aug 28 '15 at 12:08

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