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As previously discussed, sex is exercise, so it's normal to feel tired after it. But the release of oxytocin also helps you feel sleepy. For men in particular, an orgasm reduces activity in the prefrontal cortex of the brain, which is really important for getting a good night's sleep. A study, reported by the Mail online, showed how the blood rush after climax depletes muscles of energy-producing glycogen — exhausting you further.