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Although the reason for this is still unclear, most people would agree that sex leads to a better night's sleep. This may be partially due to oxytocin, which can reduce stress, leading to relaxation and sleepiness. A hormone called prolactin is generally highest when we're asleep but it also surges during orgasm and may add to the desire to drift off. Sex can also be an indicator of a healthy relationship between partners and that added support (and reduced stress) can contribute to better sleep.