What happens to you when sleep? Despite the fact that we spend a good third of our lives snoozing, most of us aren’t remotely aware of the fact that at different times of the night, we’re experiencing different stages of sleep. And if you’re awoken during the night by, say, a thunderstorm, how tired you are the next day depends in large part on when you were woken up.
Confused? Don’t be: While sleep is an unbelievably complex science, and a night of sleep comprises multiple distinct stages, there are really just three that you need to know about:
NREM STAGE 2
This stands for “non rapid eye movement sleep, stage 2,” and comes right after the first phase of sleep, which is the lightest (so light, in fact, that people aroused from this stage often think they were completely awake). NREM 2 is usually a relatively shallow, dreamless sleep that lasts around 20 minutes, but it’s important because of two funky effects:
The first is called K-Complexes, which are variations in brain waves that help the mind ease into a deeper sleep by weakening the brain’s response to stimulus. Relieved of processing any external happenings, your neurons are able to “reboot,” consolidating the day’s memories and strengthening important lessons. 
Second are “spindles,” short bursts of high frequency brain activity. This element of NREM2 helps to not only integrate important information into the brain’s memory centers, but also retain learned muscle movements—something of particular importance to athletes.  
NREM STAGE 3
Once upon a time divided into stages 3 and 4, NREM 3 is what we call slow-wave sleep (SWS) and also goes by the names of deep sleep and delta sleep. And while, ironically, it isn’t actually the deepest sleep possible, there are a lot of benefits that are unique to slow-wave sleep.
Firstly, SWS is when the body produces the most prolactin, a hormone that boosts the immune system and manages inflammation.  Slow wave sleep also suppresses the production of cortisol, which is known as “the stress hormone”—and for good reason, as it can cause anxiety, digestive problems, and even muscle breakdown. Studies have shown that poor quality sleep has a direct effect on cortisol levels, with one group of subjects experiencing a 37% increase in the hormone when they had only slept for 4 hours, and another study showing a 50% to 80% increase in cortisol among pilots who spent a week sleeping for 6 or fewer hours per night instead of their usual 7.8 hours.  
One of the reasons this cortisol suppression is significant is its relationship to what is sometimes considered the hormone’s antidote: Growth hormone. A powerful cocktail that promotes fat loss and muscle gain, the surge in GH during deep sleep allows the body to patch the wear and tear of the day by helping to repair micro tears in muscles and make them stronger than they were before.
Since it’s also been shown to improve bone density, exercise capacity, and longevity, slow wave sleep is compelling proof that a lot of 20 minute naps is no substitute for a long, restful sleep—75 percent of the body’s production of growth hormone takes place during slow wave sleep.   The body needs to reach the stage of deep sleep, and this is confirmed by the fact that people who are deprived of sleep experience a “rebound” effect and find themselves having more slow wave sleep than normal as the body tried to compensate for the deep sleep it missed.
The average sleep “cycle,” during which one passes through all four sleep phases, lasts around 100 minutes, and rapid-eye movement sleep occurs about 70 or 80 minutes in.
There are a lot of benefits that take place during REM that also occur during NREM Stage 2 and 3, like the internalization of the important memories and skills (this is an underappreciated reason why sleep quality is critical for athletes). But there are plenty of unique effects as well—for one, REM sleep subdues the amygdala, the center of the brain responsible for aggression, which is one reason why people who are low on sleep can be a little more hostile than usual.
REM sleep is also when the most intense dreaming takes place, which might be why REM has a profound effect on the areas in the brain responsible for creativity and insight.In stark contrast to quiet rest and non-REM phases, rapid eye movement sleep has been shown in multiple studies to “enhance the formation of associative networks and the integration of unassociated information.” This means that REM specifically helps with creative problem solving, and with drawing conclusions from seemingly unrelated and abstract information.  
All of this information certainly puts a new spin on the phrase, “Let me sleep on it.” Stepping back from a problem and getting a good night’s sleep literally helps the brain to put together the pieces of a puzzle, connect both unrelated and interrelated information, and help you find solutions to complex problems.
No stage of the sleep cycle should be neglected, and nor should sleep as a whole: The body, in fact, tends to concentrate slow wave sleep at the beginning of the night and more REM toward the end, so splitting up your sleep into naps or even two or three longer chunks simply won’t allow the full benefits of a night’s sleep to sink in. For inflammation, creativity, problem solving, muscle gain, bone density, improved athleticism, and countless other reasons: You need a darn good night’s rest.