1. As you fall asleep, you jerk. Every time. According to sleep experts, the degree of the jerk is suggestive of how sleepy you are, so the bigger the jerk, the more tired your body.
2. Your body temperature goes way down. When we’re active during the day, we burn more calories, so lowering temperature is a way to reduce the burn rate and save calories. “It’s like how bears hibernate,” says Dr. Avi Ishaaya, a sleep specialist and assistant clinical professor of medicine at UCLA. “Sleep is a survival mechanism.”
3. You do a full toxin cleanse. This allows the body and brain to rejuvenate. In people who don’t sleep well, the filtration isn’t as effective, so experts say that may help explain why people who are sleep-deprived can go a little crazy.
4. You forget useless information. “We take in all this information all day long, and most of it is luckily forgotten,” says sleep specialist Christopher Colwell at UCLA School of Medicine. “If you remembered everything, it would fill up your brain, so a sorting process takes place during sleep.”
5. Your body becomes paralyzed, but your brain is its most active. The brain of someone who is dreaming is actually more active than that of a person who is awake and requires more oxygen. “One theory is that in REM sleep, you’re organizing thoughts and learning, filing information, but no one really understands specifically why a sleeping mind is active,” says Dr. Alon Y. Avidan, a professor of neurology and director of UCLA’s Sleep Disorders Center. Your mind is so active during sleep that it can even make important connections and discoveries, researchers say.
6. All of your individual cells—not just your brain cells—are being completely repaired. These restorative processes take care of all the damage that’s occurred during the day. When you don’t get enough sleep, the effect doesn’t just zap your brain—it impacts your whole body. Hence why you feel like you’ve been beat up after an all-nighter.
7. Your immune system is at an all-time high. One study showed that people who received flu shots and were sleep-deprived the next night did not create the antibodies required to protect against the flu. “As soon as I see the first signs of an upper respiratory infection, I try to sleep for ten hours,” says Dr. Jordan Stern, founder and director of the BlueSleep Center in New York City. “If you’re chronically sleep deprived, you’re more likely to develop recurring infections.”
8. You wake up between 5 and 15 times per hour. It usually happens when we shift to different stages of sleep, like from dreams to deep sleep, and the awakenings happen so briefly that we don’t remember them.
9. You’re probably breathing–a fact most of us take for granted during sleep—but up to 30 percent of people actually stop breathing at night. Up to 90 percent of people with this disorder, sleep apnea, go undiagnosed, including young women. The life expectancy for someone with untreated sleep apnea is only 58 years, so if you or your partner snores at night, it’s time to get checked out, says Dr. Stern.
This article originally appeared onCosmopolitan.com. Minor edits have been made by the Cosmo.ph editors.