Have you ever wondered why we sleep? Is it something we really need? We spend a third of our lives asleep. Isn't that a waste of time? These were big questions for scientists two decades ago, but now, thanks to research and experiments we already know the answers and we can take advantage of that knowledge and tap into the power of slumber.

First things first, why do we actually need to sleep? To answer this question we need to divide the slumber into phases as each one plays a different role. The first phase is called NREM (non-rapid eye movement) and it's a phase of deep sleep when our body and brain rest. The second phase is called REM (rapid eye movement) and it's also called a dreaming phase as during it our imagination and memories come into play. Both phases are vital and have unique functions when it comes to maintaining the brain's overall cognitive performance. Typically, once you fall asleep after a couple of minutes of shallow slumber you enter the NREM phase. Each NREM phase is followed by the REM phase to complete the cycle. The whole cycle (NREM + REM) takes from 90 to 120 minutes. In the first cycle NREM phase is dominant but as the night goes by REM is usually taking over and the closer to awakening the more we dream. There are no mistakes when it comes to biology and just by looking at the cycles and how their duration changes we can tell that deep NREM sleep is the most important one for our brain's cognitive function and therefore it dominates the first cycles.

Let's take a closer look at what happens right after we close our eyes and how the NREM phase progresses. The first stage is simple, you keep your eyes closed, but you're still awake. Typically, you recall some matters from the day or think about the following one. Your heartbeat and breathing slow down. Your muscles become relaxed and at the end, you fall asleep. It's easy to interrupt the process as you are still getting signals from the outside through your senses. This stage might last between 5 to 15 minutes depending on many factors e.g. melatonin level, how exhausted you are etc. Before we move on to the next phase, let's stop here for a moment as something magical sometimes happens here. This magic indeed has a name - hypnagogic hallucinations. As you become relaxed and just before falling asleep you might experience these vivid sensations. You might hear, feel, see or even smell something. It can happen while transitioning to sleep or the opposite. These events are usually brief and most of us either don't experience them or don't pay attention to them, but as I experienced them I can assure you they're strange and wonderful at the same time. Pay attention to them next time you go to bed, it's definitely worth it.

In the next phase, you're already sleeping, but it's still light slumber and therefore small disturbance might wake you up. Your heartbeat, blood pressure, and breathing continue to slow down. The body temperature drops and eye movements stop. You become less aware of your surroundings. The brain starts to generate rhythmic brain waves called "sleep spindles" whose role is to gather and process all the memories you amassed the previous day. This phase takes about 20 minutes and its role is to get your body ready for the next one - deep sleep.

As you enter the final stage your heartbeat and breathing reach their lowest rates and your body is fully relaxed. All the external stimuli are blocked and therefore it's hard to rouse you, and if someone wakes you up you feel disoriented for a couple of minutes. Delta brain waves begin to emerge and the clean-up process begins. The brain gets rid of all the substances that were amassed during the previous day. In fact, these substances are somehow poisonous, so it's extremely important not to neglect everyday sleep as you don't want to intoxicate yourself.

It reminds me of an experiment that I read about in another book. Scientists wanted to pinpoint what caused the death of mice deprived of sleep. Long story short, it turned out that a certain type of bacteria, which is normally in check, took over the intestines and eventually caused fatal intoxication that completely wrecked the liver. It won't happen after one night of sleep deficit, but it should give you an idea of how important the clean-up process is.

Another important function of deep NREM sleep is information transition. This is when the magic happens and all information temporarily kept in short-term memory is moved and persisted in the long-term memory. The way it happens is strongly correlated with delta brain waves. There are many studies showing that the learning process is effective only if we're provided with enough time to sleep. On contrary, sleep-deprived people are much more likely to forget the data that they'd been trying to remember. Taking that into account, cramming before the exam doesn't make sense if you lack sleep.

The functions of deep NREM sleep I mentioned above are not the only ones. Apart from these delta sleep facilitates many regeneration processes like tissue growth and repair as well as cell regeneration. Additionally, it's proven that NREM slumber plays important role in immune system strengthening. There are probably more features I didn't mention and probably even more yet to be discovered, but even though the conclusion is simple - sleep is indispensable, and therefore you should prioritize it. And one more thing, this is just the first part that covers deep REM sleep. There's another one, even more wonderful, pertaining to the dreaming phase, so stay tuned.