Why are we asleep?

Scientists are still unsure - Why do we sleep? But, some facts have already come to light, lifting the veil over this mystery. He who sleeps... recovers: here is everything you ever wanted to know about sleep, from its stages to theories about its basic functions
We spend a third of our lives asleep: 24 years with our eyes closed, doing nothing. From an evolutionary point of view, this doesn't seem like a very clever strategy: while animals are vulnerable to predators in their sleep, there isn't a single living thing that can do without rest. Sleep must be useful for something, but what exactly is it?

Sleep is a nocturnal 'cleaner' and some believe that the CSF (central nervous system fluid) cleansing system, which cleanses the brain of toxins accumulated during the day, is particularly effective during sleep. If not properly disposed of, these substances accumulate and can contribute to neurodegenerative diseases such as Alzheimer's disease. These preventative 'spring clean' works best when the brain is not busy with daytime activities.

Another popular theory states that sleep serves to consolidate memory, i.e. to consolidate the most important concepts that we have learned during the day, and to discard unnecessary ones. In other words, sleep promotes memory and stimulates creative thinking: on the one hand, it reorganizes the information analyzed by the brain into separate categories, and on the other creates new connections between new concepts and those already learned. That's why a good night's sleep reminds us of words we had forgotten the night before, or why we wake up inspired by a brilliant idea!

Restorative sleep

Finally, there are those who believe that while sleeping, the body produces substances very important to it, such as growth hormones and certain proteins that serve for tissue repair. The fact is that when, for whatever reason, a person does not get enough sleep, the consequences do not last long.

One night without sleep is enough to feel the first negative effects on mood and ability to concentrate. If the lack of sleep increases, language, memory, sense of time, ability to plan and make judgements, and reflexes in response to rapidly changing situations are affected. After 17 hours without sleep, there is a decline in cognitive ability similar to that seen after two large glasses of wine.

Chronic sleep deprivation can contribute to cardiovascular problems, obesity, diabetes and some cancers. The problem lies not only in the amount of sleep, but also in the regularity of sleep-wake rhythms dictated by the biological clock: even those who are forced to work at night and sleep intermittently can report negative health effects.

Sleep and how do we sleep?

The need for sleep after many hours spent awake is what scientists call homeostatic sleep pressure. Another mechanism that makes us want to go to bed is the onset of night: our circadian rhythms mean that we feel more tired as darkness falls. Sleep consists of repetitive cycles of about 90 minutes and is divided into two macro-categories: REM sleep and non-REM sleep. Non-REM sleep, on the other hand, is divided into four stages. The first is light sleep, when we fall asleep and are in a half-asleep state. Muscles begin to relax and sometimes "sharply contract"; we easily wake up at the first sound. After 10 minutes, the real sleep stage begins, when the heartbeat and breathing slows down.

We then move on to stages three and four, deep or slow-wave sleep (based on the type of electrical activity produced by the brain). This is the calmest stage, during which breathing and heartbeat reach the most relaxed rhythm. Muscles move very slightly and if you are awakened during this time, you feel disoriented for several minutes.

These stages are interspersed with REM-sleep (from Rapid Eye Movement) phases, which can be recognised by the rapid movements of our closed eyes. The REM phase occurs three to five times a night and is characterised by intense brain activity. Most dreams occur during this phase, when our heart rate and blood pressure are elevated, but our body is virtually paralysed: a form of protection that prevents us from moving as we do in the dream we are dreaming.

Why do we sleep and how much should we sleep?

In general, sleep is a very active process in which the brain is at rest, and the calorie expenditure is also significant: the amount of energy we save by sleeping for eight hours is equivalent to 50 kcal, almost a few bites of toast! An adult needs on average 7-8 hours of sleep to lead a healthy life. School-age children need 9 to 11 hours, and teenagers 8 to 10 hours a night.

As we age, we sleep less, possibly due to a reduction in the number of brain cells that 'tell' the brain when to sleep. The quality of night sleep also decreases with age, so older people often feel the need to catch up on their afternoon naps. Genetics also play a role in determining how many hours of sleep we need: some people don't feel rested with less than eight hours a night, while those who get up with six and a half already feel energetic.

Those who have difficulty falling asleep can try a few simple steps. The main one is to avoid using a smartphone shortly before bedtime: the blue light from electronic devices suppresses the production of melatonin (the sleep hormone). Even late-night strenuous sports can prevent you from falling asleep: it's better to plan for them before dinner and then devote yourself to quieter activities.