Anyone who's ever shared a bed with a guy can tell you that men and women don't always see shut-eye to shut-eye when it comes to sleep—regardless of your daytime (or sexual) chemistry. Here's why:
1. Women get tired earlier in the evening.
Your circadian rhythm is one system that moderates your body's natural sense of when it's supposed to sleep and wake up. Among women, this system starts to spew out signals for bedtime about two hours earlier than it does for men, according to Diane Boivin, MD, director of the Centre for Study and Treatment of Circadian Rhythms at McGill University's Douglas Mental Health University Institute, where she recently coauthored a study on the topic.
2. Men have more trouble falling asleep early.
It's no wonder he tosses and turns when you both hit the sack at your bedtime: Men's sleep cues kick in roughly two hours later than women's. And when your natural clocks effectively operate as if you're stationed in two different time zones, you're unlikely to fall asleep at the exact same time—particularly when he goes to sleep at your bedtime.
3. Women are more likely to wake up before the alarm goes off.
Besides your circadian rhythm, another system known as homeostasis—the urge to sleep that acquires throughout the day—moderates your sleep. The only problem is that after your first few hours of shut-eye, this drive to sleep goes away, at which point it's up to your circadian rhythm to keep your eyes closed until morning. But remember: Most women’s circadian rhythms are two hours ahead of men's, so women are naturally more alert earlier in the day. It could be why you find yourself wide awake at the crack of dawn while your bed partner snores on.
4. Men are better at sleeping in.
Just because men feel ready for bed later than woman doesn't mean guys need less sleep overall. Their bodies just make up for late nights by sending the signal to sleep late into the morning, according to Dr. Boivin. This could explain why you manage to make a Starbucks run, clean your entire apartment, and finish an episode of KUWTK by the time your sleeping partner even begins to stir.
5. Men dream more than women.
Dreaming typically happens during the deepest phase of sleep known as REM. Your ability to fall and stay asleep depends on a bedtime drop in body temperature, and higher levels of progesterone during the second part of the menstrual cycle cause women’s body temperature to rise particularly at night. That means women who regularly ovulate are likely to have more trouble sleeping and rack up fewer hours of REM during the weeks leading up to their periods, according to Dr. Boivin—all of which is to say you get less time for dreaming. It doesn’t help that the body secretes less melatonin, the sleep hormone that plays a major role in regulating your circadian rhythm, during the second half of your cycle, which messes with your REM sleep even more. And considering the fact that you can experience normal menstrual cycles for some 40-plus some years, this sex difference in dreaming can seriously accumulate.
6. Women are more prone to having cold feet in bed.
Experts say the nerves that control blood flow to the hands and feet are more sensitive in women than in men—so when your body temperature tries to drop, as it’s wont to do when your circadian rhythm decides it’s time for sleep, blood vessels constrict, which slows the flow of warm blood to extremities. This leaves your feet freaking F-R-E-E-Z-I-N-G—but stalls the body's temperature change, which ultimately messes with your ability to fall and stay asleep, according to Dr. Boivin. Yeah, it's annoying, but at least you've got a solid, science-backed excuse for hogging the covers.
Of course, all this research is based on biology—if it doesn't ring a bell for you and your sleep partner, it's probably because you don't follow your natural sleep cycle in the first place, which would involve sleeping when your body says to sleep and waking up without an alarm.