Researchers in 2013 discovered that the glymphatic system shifts into overdrive when you sleep. Your glymphatic system is what cleans out waste from your nervous system, using the cerebrospinal fluid. In their study, scientists discovered the spaces between brain cells expands and contracts up to 60% during sleep while more interstitial wastes are cleared out. This study was the first direct evidence that your brain cleans itself while you sleep.

The brain lacks the ability to differentiate what happens in your dreams from what happens in real life. This means that unless something is done, you'll act your dreams out. Because of this, your body practices REM atonia, where it uses chemicals to cut off signals from your brain to most of the body -- except for regulatory functions. Occasionally some signals get through, causing twitches and small movements. However, for the majority of your time asleep, your brain has you paralyzed.

Sleep spindles are a sudden and intense burst of brain activity, followed by a K-complex (like an "all clear" signal of brain waves). A sleep spindle will cause sudden twitches and movements in your limbs, and often occurs more often in younger children. It is thought that sleep spindles are your brain's way of learning what signals control what parts of your body. It's kind of like an "I wonder what this button does" activity for your brain. It's completely normal, so relax.

As you enter the first stages of sleep, your heartbeat and breathing rate starts tapering off. This is because most of your body has clocked out for the night and doesn't really require anything. With the majority of stuff from the neck down on autopilot, your brain is the only thing sucking up significant energy. Because of this, your heart beats less and you don't breathe as much.

Scientists have shown sleep accelerates the wound healing process, and keeps the immune system from getting lazy. This is supported by the fact that your metabolism slows way down while you sleep, delivering less oxygen to cells. This signals them to stop doing what they do, and start repairing and restoring. At the same time, many of the synapses in your brain shift from "doing stuff" mode, to a state of restoration and maintenance.

It's called Nocturnal Penile Tumescence, and it happens more than just the morning. In fact, most healthy men will experience 3-6 erections in a night. No matter what you heard, it isn't related to a full bladder. You just get it because you get it, and science isn't 100% on why. Even weirder is Nocturnal Clitoral Tumescence, which is the same concept just with a clitoris instead of a penis. Women will experience a swollen, aroused clitoris with the same frequency as NPT. Again, science doesn't have any... wait for it... hard evidence for a cause.

REM stands for Rapid Eye Movement, and is also a really great band. In this super-deep stage of sleep, your body stops regulating breathing and body temperature and then silences all alarms that let you know you're too cold or choking. While the body is coasting in neutral, your brain is all over the place dreaming. The movement of your eyes during REM sleep is thought to correlate to stuff going on in your dream, but for some reason follows a cyclical, "loop" pattern.

When you sleep, it affects your "sensory threshold." This doesn't mean you feel less, it just means your body cares less about it. It changes the point at which stimuli will prompt your body to do anything about it. Sudden, loud noises will still wake you, but a small nudge might not. This is because your body turns the "give a crap" dial way down. Why some people are "deep" sleepers and some are "light" is not fully understood.

Sleep is absolutely tied to memory function, and many studies have proven the link between proper sleep and a functioning memory. How that works is up for some level of debate, however. Some have said that the dendrites in your brain that help with memory are refreshed and cleaned out when you sleep. Others have suggested that your brain flips through memories like a file cabinet -- which is why you dream. There's even a theory of "memory consolidation" that suggests your sleeping brain tosses stuff out you don't need.

During REM Sleep, your brain releases huge amounts of chemicals that encourage creativity and reason. Because of this, your mind becomes “hyper-associative." What this does is amplify the effects of semantic priming, which is a Jedi Mind Trick you didn't know you have, allowing you to better associate certain words when preceded by certain words. People who have been awoken from REM Sleep and given anagrams or creative problem solving, after punching the researchers in the throat -- perform much better.

The endocrine system is responsible for spitting out hormones at all the right times to all the right places, thus ensuring stuff gets done. A dash of this, a pinch of that, and you're all set to go. There's a whole batch of hormones that are sleep-dependent and must be distributed while you're out. Growth Hormones in particular, as well as prolactin (regulates lactation) are sprayed out when you sleep. Melatonin regulates your internal clock (and is not a sleep aid) also gets distributed. Cortisol is important for your metabolism and you double-down on the stuff in REM Sleep.

There are more theories for why we dream than there are dreams. However, one pretty interesting theory says your dreams are your brain practicing certain situations to prepare for them happening in real life. Two studies in 2000 in the journal of Behavioral And Brain Sciences proposed a model that the brain uses dreams to see how you react to threatening situations, so you're better prepared in the real world. The theory goes on to say that "genetic memories" that are in our mind from birth contain many of these practice scenarios -- like a "starter pack." Which explains why people often have similar dreams.

Neural oscillations, or brain waves, are created by the electrical signals in your brain. Certain waves correlate with certain activities. Scientists are aware of them, and can use them to measure stuff, but have absolutely no clue what purpose they serve. When you sleep however, your neural oscillations become oddly specific for each stage of sleep. Alpha activity, theta waves, delta waves... they all are emitted during very specific points in your sleep cycle. So much so, that scientists can pinpoint the exact part of sleep you're in -- just from the type of brain wave given off.

Your brain gets more oxygen and nutrients, and you can blame it upon a rush of blood to the head. (No Coldplay fans out there? Really?) Since your brain is one of the only parts of your body that's still fully functioning, what blood is still being pumped gets diverted to your head. This powers all the maintenance functions going on in your skull.

The neurotransmitters norepinephrine, serotonin and histamine are kind of like the cocaine, molly, and PCP of your brain -- respectively. They do a lot of important things when you're awake like sharpen your senses, make you feel good, and help regulate the immune system. During REM Sleep, all three of these neurotransmitters are switched off. Your brain is denied access to all three of them in favor of acetylcholine, which activates muscles and movement. The release of acetylcholine is thought to speed up brain waves -- maybe.