This Is What Happens to Your Brain When You Sleep

It's 11 p.m. and your insanely busy day is finally coming to a close. The kids are in bed, tomorrow's lunches are packed and you've laid out your work clothes for the morning. Your head hits the pillow and you barely squeak out a "goodnight" to hubby before you drift off to dreamland. In a few hours you'll wake up and repeat. Ever wonder what goes on during those hours when your brain is seemingly turned off for the night? Prevention has you covered. Check out these 10 weird facts about sleep below.

You aren't sleeping deeply most of the time. Not all sleep was created equal: When you first drift off, you get only very light sleep, then progress deeper and deeper into dreamland. The sleep cycle starts in what's called non–rapid eye movement or NREM stage 1 (the kind of sleep you might nab if you were the type to doze off during your college lectures; you know who you are). Then you move into a deeper NREM 2 and then to the deepest, NREM 3, also called slow-wave sleep. Finally, you land in rapid eye movement, or REM, sleep, the wild part of the ride when most of our dreams occur. The whole shebang usually takes somewhere between 90 and 120 minutes, so on a typical night you'll cycle through four or five times, waking up for just a sec (even if you don't realize you're awake) after REM sleep before starting over in stage 1.

As the night goes on, you spend less time in that deliciously deep stage 3 and more time in REM sleep, which explains why your alarm so often wakes you up in the middle of a totally bizarre dream, says Sigrid C. Veasey, MD, a neuroscientist and a professor of medicine at the University of Pennsylvania's Center for Sleep and Circadian Neurobiology. But we don't really know why REM periods get longer in the wee hours, says Daniel A. Barone, MD, an assistant professor of neurology at the Weill Cornell Medical College's Center for Sleep Medicine. One theory, he says, is that REM sleep may somehow prepare you to get your butt out of bed.

Your brain cleans house. Our brains are "on" throughout the night, especially in that dream-heavy REM sleep, Barone says, when they're actually almost as active as they are when we're wide awake.

Among other things, they may be taking out the trash. That's one of the more exciting new ideas about the purpose of sleep: A 2013 study in mice found that waste removal systems in the brain are more active during sleep. Perhaps, the researchers theorized, we sleep to allow time to clear away toxic byproducts that would otherwise pile up and cause problems, like the trademark plaques of Alzheimer's disease, Veasey says.

Your brain's also busy cementing new memories while you sleep. "We think the brain is processing the information we gained throughout the day and filtering out the information we don't need, which may be one of the reasons we dream," Barone says. The theory goes that maybe connections between brain cells are strengthened or weakened during sleep, depending on how much we used them during the day, he says. The important stuff gets reinforced while the factoids we just don't need get trashed.

>> Read more: 20 Weird Facts About The Human Body... Your Body!

Your heart rate and breathing slow. That "can't…move…another…muscle" feeling comes from the fact that all sorts of normal physiological processes slow way down at bedtime, like how many breaths you take per minute and how quickly your heart beats. Even your muscles and organs chill out. "The intestines quiet down in the nighttime, and the liver goes from trying to detoxify during wakefulness to trying to build and synthesize when you're sleeping," Veasey says. There's also less adrenaline pumping through your veins, since you won't be needing your fight-or-flight response between the sheets (at least, we hope).

Your blood pressure plummets. Total-body relaxation results in something called a "nocturnal dipping" of your blood pressure, Veasey says. If you're otherwise fit, your blood pressure can drop by about 5 to 7 points with a good night's sleep. (Got high blood pressure when you're awake? Check out this list of foods that lower blood pressure naturally.) And so does your body temperature. Sleep experts are constantly quoted in articles like these saying to keep your room cool for a good night's sleep. But they're not just saying it because it sucks to try to fall asleep with your hair plastered to your neck with sweat. A cool room actually mimics something your body's doing naturally: While we sleep, core temperature drops a bit, so cooling off before bed can help you nod off.

During REM sleep, you might chill by a whole degree or 2. "If you were cold and you were awake, you would shiver, but during REM sleep the body loses its capacity for thermoregulation," Veasey says, "and we have no earthly idea why that happens."

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