Sleep: how much do we really need? | Science

Why are we sleeping?

"The only known function of sleep is to cure sleepiness," joked Dr. J Allan Hobson, a science scientist in sleep science at Harvard. That's not entirely true, but the question of why we spend about a third of our lives sleeping and what's going on in our minds at this point is far from resolved.

A big mystery is why sleep has become an evolutionary strategy. It must confer powerful benefits to balance substantial risks, such as being eaten or running out of food while sleeping. Research shows that sleep is not a luxury but an essential condition for physical and mental health. But the complex and diverse functions of sleep are just beginning to be discovered.

What's going on in our brains while we sleep?

The brain does not simply go out. It generates two main types of sleep: slow-wave sleep (deep sleep) – SWS – and rapid eye movement (dreaming).

About 80% of our sleep is SWS, characterized by slow brain waves, relaxed muscles, and slow, deep breathing. There is strong evidence that deep sleep is important for memory consolidation, with recent experiences being transferred to long-term storage. However, this does not happen indiscriminately – it also seems to be necessary to remove the least relevant experiences of the previous day. A study published last year found that connections between neurons, called synapses, contracted during sleep, which resulted in the elimination of the weakest connections and the forgetting of those experiences.

Dreams represent the remaining 20% ​​of our sleep time and the duration of dreams can range from a few seconds to over an hour. Dreams tend to last longer as the night progresses and most are quickly or immediately forgotten. During REM sleep, the brain is very active, the muscles of the body are paralyzed, the heart rate increases and breathing can become irregular. It is also thought that the dream plays a role in learning and memory – after new experiences, we tend to dream more. But that does not seem to be crucial either: the doctors discovered that a 33-year-old man who had little or no REM sleep due to a brainstem burst injury had no memory problem important.

How much sleep is enough?

We often talk about eight hours, but the optimal sleep time varies according to people and at different times of life. As part of a comprehensive review, in which 18 experts reviewed 320 existing research articles, the US National Sleep Foundation concluded that the ideal sleep time is seven to nine hours for adults and eight to ten hours. for teenagers. Younger children need a lot more, newborns requiring up to 17 hours a day (not always in step with the parents' sleep cycle).

However, the experts did not take into account the quality of sleep nor the amount of SWS v REM sleep. Some people survive with less sleep because they sleep well, but under seven, there was compelling evidence of adverse health effects. According to experts, too much sleep is also bad, but few people seem to be affected by this problem. In the United Kingdom, the average sleep time is 6.8 hours.

What about shift work – does it matter when you sleep?

In the 1930s, an American scientist, Nathaniel Kleitman, spent 32 days at 42 meters underground at Mammoth Cave, Kentucky. The goal was to study the biological clock. Living in complete isolation, without any outside sign of night and day, he adopted a 28-hour day. Kleitman did not adapt and was awake only when his assigned "day" coincided with the day in the outside world.

His body temperature also continued to follow a cycle of nearly 24 hours. Many shiftworkers, especially those working irregularly, face similar problems. In recent years, this issue has been taken more seriously, with professional sports teams hiring consultants for training and travel schedules abroad. The US Navy has modified its post system to align with the 24-hour clock, rather than the 18-hour day used in the old British system.

Why are we stuck on this 24 hour cycle?

Over the course of millions of years of evolution, life has become deeply synchronized with the day-night cycle as our planet rotates. The so-called circadian rhythms are evident in almost all life forms and are so firmly etched in our biological mechanism that they continue even in the absence of any external input. Plants kept in a dark cabinet at a stable temperature open and close their leaves as if they could feel the sun without seeing it.

In the 1970s, scientists uncovered a mechanism essential to this recording of internal molecular time. In experiments on fruit flies, they found a gene, later called "period", whose activity seemed to rise and fall reliably every 24 hours. Scientists, two of whom were awarded the Nobel Prize last year, then showed that the gene of the period was working by releasing a protein that accumulated in the cells overnight, before it was released. to be degraded the same day.

Later, it was shown that humans had the same gene, expressed in a small area of ​​the brain called the suprachiasmatic nucleus (NSC). This serves as a channel between the retina of the eye and the pineal gland of the brain, which pumps melatonin, a sleep hormone. So when it's dark, we're sleepy.

So is it just our brain that is affected?

The SCN clock is the chronometer of our body, but in the last ten years scientists have discovered that clock genes are active in almost every cell type of the body. The activity of about half of our genes appears to be under circadian control.

The activity of blood cells, liver, kidneys and lungs in a petri dish increases and decreases in a cycle of about 24 hours, and virtually everything in our body – from the secretion of secretions. hormones in the preparation of digestive enzymes in the intestine, changes in blood pressure to body temperature – is influenced in a major way by the time of day when these things are normally required.

The link between the ticking of each neuron and the more complex rhythms of brain waves that appear in our brains during sleep is not yet clear, but scientists are currently conducting an investigation. When brain cells are cultured in a laboratory dish, they begin to self-organize and begin to show, in rather agonizing fashion, patterns of activity similar to those seen during the sleep.

Have we slept more deeply in the past?

Lack of sleep is often seen as a modern problem, as a scourge of sedentary lifestyles and as stuck to smartphones late at night. However, research on the sleeping habits of modern hunter gatherers suggests that this may paint a too romantic picture of the past. A study of Hadza in northern Tanzania revealed frequent nighttime awakenings and very different sleep schedules among individuals. Over a period of three weeks, there were only 18 minutes left when the 33 members of the tribe were sleeping simultaneously. The scientists behind the work concluded that restless sleep could be an old survival mechanism designed to protect oneself from night-time threats.

The main difference seemed to be that the tribe's members were free from the paranoia and anxiety caused by sleep problems, which is a source of common concern in Western countries.

What happens when you are not sleeping enough?

In extreme cases, sleep deprivation can be fatal. Rats that are completely sleep deprived die within two or three weeks. Of course, this experience has not been repeated in humans, but even a day or two of sleep deprivation can cause hallucinations and physical symptoms in healthy people. After a bad night's sleep, cognitive abilities have an immediate impact. Concentration and memory are significantly affected and people are more likely to be impulsive and prefer instant gratification to the expectation of a better outcome. We are also worse when we are tired – a study found that people who are deprived of sleep are more likely to cheat and lie.

What about physical health?

Lack of cumulative sleep can have long-term health consequences and links are seen with obesity, diabetes, heart disease and dementia. Last year, a review of 28 existing studies found that permanent night workers were 29% more likely to develop obesity or become overweight than rotational workers. Results based on more than 2 million individuals showed that night shifts increased the risk of heart attack or stroke by 41%.

The reasons for some of these associations are complex and difficult to separate from lifestyle factors. The studies mentioned above have tried to filter out socio-economic factors, for example, but factors such as stress and social isolation may be more difficult to capture. That said, there is more and more evidence of a direct biological influence. Sleep deprivation has been shown to alter the body's basic metabolism and the balance between fat and muscle mass.

Insomnia has long been recognized as a common symptom of dementia, but some scientists also believe that sleep disorders may play a role in Alzheimer's disease. Research has shown that the brain "cleans" itself during the sleep of amyloid-related proteins linked to Alzheimer's disease and that sleep deprivation leads to an increase in the levels of these toxins.

Do all animals sleep?

The answer depends in part on what counts as sleep. Most scientists seem to accept the definition of (a) being motionless and (b) being significantly less sensitive than when awake. On the basis of these criteria, there have been some candidates for sleepless species, but none have been conclusively demonstrated to be true. The Bullfrog was one of the first adversaries – a 1967 study revealed that frogs reacted the same way when they received electric shocks during the day and in the middle of the night. But doubt has since been cast on this discovery. There are animals that require little sleep: it is thought that the little bat will sleep 19 hours a day; Adult giraffes rarely sleep more than five minutes at a time. And sleep is not the same for all animals. Dolphins have the ability to sleep only half their brain at a time, called unihemispheric sleep. Migratory birds are thought to be sleeping and sharks are sleeping and swimming.

Reading list

The house of sleep. Jonathan Coe

Why we sleep, Matthew Walker

Overcoming the problems of insomnia and sleep, Colin Espie

Sleepfaring: A journey through the science of sleep, Jim Horne

Sleepyhead Henry Nicholls