Quite a strange idea: Every night we lie down waiting to go into some sort of unconscious state, trusting that we will wake up about 8 hours later. This is what we call sleep. We sleep 1/3th of our lives, that is on average 26 years! But how do we get into such a sleep state? And what good does it actually do? 

It used to be thought that the brain simply "turns off" during sleep, but this is not true. A whole lot of

things happen. When it gets dark, the pineal gland, a gland in the center of the brain that looks like a very small pinecone, activates. This produces melatonin, a substance that makes you sleepy. Light causes inhibition of the pineal gland, then melatonin production stops. This is why we feel tired at night and (hopefully) not during the day. 

The brainstem (including the pons, medulla, and midbrain) and the hypothalamus produce GABA, an inhibitory neurotransmitter. This neurotransmitter causes other neurons to fire less frequently and therefore puts the brain in a calmer state. With an electroencephalogram (EEG), we can measure this change in brain activity. 

During sleep we go through different stages. We first enter non-REM sleep. On the EEG, the frequency of brain activity slowly decreases and the amplitude increases. This means that neurons are firing synchronously in a slower rhythm. And that's how we enter deep sleep. The thalamus, the gatekeeper of the brain, no longer lets signals through to the cortex, isolating you from the external world. Then brain activity accelerates again as we enter REM sleep. Interestingly, during REM sleep the EEG shows the same activity as when you are awake. This is because we are dreaming. The thalamus then sets the gate ajar allowing signals to go to the cortex. These signals fuel the content of our dreams. The brainstem sends inhibitory signals to the muscles so that we do not actually execute the movements in our dreams. Our whole body is paralyzed during REM sleep, except for the eyes that do move during dreaming. That explains the term "rapid-eye movement" sleep. This cycle from non-REM to REM sleep lasts ±90 minutes and repeats itself about 5 times in one night. 

There are books filled with explanations about the mechanisms behind sleep, but this, in a nutshell, is how it works. That leaves the question of why we actually sleep. In fact, evolutionarily speaking, sleep seems like a waste of time. You would think that an organism would be better occupied gathering food or finding a mate, rather than sleeping 1/3rd of the day. There are several theories about the usefulness of sleep. One explanation is that it is a kind of eco-mode of the body to save energy, since we burn 10% fewer calories during sleep. Also, our body recovers during sleep. After all, sleep strengthens the immune system and growth hormones are released. In addition, during sleep the brain is "cleaned" of waste products accumulated during the day. Changes also take place between connections in the brain. This is called neuroplasticity. Among other things, this allows us to store information in memory more efficiently. The saying “let me sleep on that” therefore makes a lot of sense! In short, sleep has many different functions and is important for our health. In fact, sleep is vital. 

Author: Pauline van Gils
Illustrations: Pauline van Gils