Dr. Mehmet Oz, co-author of You: The Owner’s Manual, calls it your “dipstick” because it reveals the health of your other body parts. The physical mechanisms that send blood to your brain, heart and kidneys also inflate your penis. If it’s not standing tall, you may have arterial problems. “A man’s lack of interest in sex is a big red flag that something is out of balance. Up to 25% of men have a low-to-no sex drive,” says Dr. Laux. “It could be even higher, as men don't talk about low libido.” The possible causes of restricted blood flow include diabetes, endocrine disorders, heart and vascular problems or kidney diseases. “But,” says Laux, “don't rule out fatigue, psychological factors, relationship issues, poor nutrition, insomnia or lack of exercise.”
It's a real and well-documented phenomenon—and it’s not just about laziness or lack of interest. There’s a fascinating biological cascade behind it:
🧠 Hormonal Shifts
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Prolactin Surge: After orgasm, men experience a spike in prolactin, a hormone that promotes relaxation and sleepiness. It also dampens dopamine, the “motivation” neurotransmitter, which helps shut down sexual arousal.
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Oxytocin & Vasopressin: These bonding hormones flood the system during intimacy. While both sexes release oxytocin, men tend to produce more vasopressin, which can contribute to drowsiness.
🏋️♂️ Physical Exertion
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Sex can be surprisingly taxing—heart rates rise, muscles engage, and men often burn more calories during the act than women. That physical effort alone can lead to fatigue.
🛏️ Environmental Cues
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The setting matters too. Sex often happens at night, in bed, and in a relaxed state—perfect conditions for sleep onset.
🧬 Evolutionary Theories
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Some researchers even suggest that this post-coital sleepiness might be nature’s way of enforcing a recovery period, or encouraging bonding and sperm retention in women for reproductive success.