The plain answer is that we do not know, but we do have some hypotheses. What it is clear is that sleeping and the brain are very closely related, so this question belongs to the field of neuroscience. Actually, sleep studies include the so-called hipnogram recordings, which includes EEG recordings. The different sleep stages are extracted from these EEG recordings.
Two interesting facts about sleep:
- We spend around one third of our lives sleeping.
- You can live longer without food than without sleep.
Indeed, sleeping is important, not to say fundamental or even better mandatory, as your parents have probably told you hundreds of times in your childhood. Randy Gardner spent 11 days (264 hours) without sleeping in 1964. Some others claim to have gone without sleep for up to 18 days, but Gardner’s case is still the most famous since it was highly documented. Although no long-term physical or psychological effects were observed, Gardner showed important cognitive impairment, paranoia and other psychological syndromes during this time. This again proves how sleep seems to be important for the good functioning of our brains. There are four main ‘why-do-we-sleep’ hypotheses. Two are related with body metabolism and the other two are brain-related. Let’s have a look at them.
While awake, our metabolism runs faster and our cells generate free radicals that are highly reactive and damage our cells. Scientists have found that during sleep the expression of the genes involved in fixing our cells increases considerably. This hypothesis agrees with the fact that animals with higher metabolic rates sleep more than animals with lower metabolic rate. Indeed some mice sleep 20 hours a day, while elephants sleep only 2-3 hours.
Our body (i.e. our cells) consumes energy and the energy currency is a molecule called ATP. When burned, ATPgenerates adenosine. Obviously we consume more energy while awake and, logically, after some time of being awake, the concentration of adenosine increases telling your body it’s time to go to bed. During sleep, the adenosine level will decrease in order to start a new day with replenished energy.
While awake, your brain is constantly absorbing huge amounts of information. As our brain is plastic, new synapses (i.e. connections between neurones) are created during the day. But our brain has limited physical space (and also limited energy) so we cannot keep on increasing the number of synapses indefinitely. Sleep time then would be the moment when superfluous synapses are cleaned out.
Glymphatic Cleansing of the Brain
This occurs at a 90% rate while we sleep and only a 10% rate while we are awake.
This hypothesis is related with the previous one. Time in bed might be needed to ‘back-up’ the relevant information learned during the day. There are experiments supporting this: the brain activity of mice in learning how to get out of a maze was similar to the one they had while sleeping. Of course it is well know that the best thing you can do the day before of an exam is to have a good night’s sleep, although few of us actually do it and instead we study overnight…
Of course, it is very likely that several of these hypotheses are true, since they are not exclusive. In any case, and as a personal opinion, I tend to incline myself to the brain related hypotheses. The reason is simple: if we lie down and relax (but without falling sleep!) we can decrease our metabolic rate and therefore we should be able to spend our entire life without sleeping. But that would not be possible because our brain would be always ‘on’ and we would end up having serious cognitive problems… please don’t try this at home!