EEvolutionarily speaking, sleep doesn’t seem like the most sensible necessity: we spend a third of our lives asleep – and thus in a potentially defenseless state. And yet sleep is essential for survival, especially our brain depends on it – not to take a break, but to store memories, to regenerate and even to be flushed.
For the participants in an experiment conducted by the US sleep researcher Allan Rechtschaffen in 1983, it was a matter of life and death: They sat on a turntable over a pool of water and sensors recorded whether they were still awake. As soon as they started to fall asleep, the plate started moving – and the subjects had to run to avoid falling into the water.
After a week and a half of sleep deprivation, they ate more and still lost weight while at the same time not being able to maintain their body temperature. In addition, they developed tumors and infectious diseases, and their hormone levels shifted dramatically. All were dead within weeks, presumably from metabolic overload, immune system collapse, and utter exhaustion.
Of course, the subjects were not humans, but rats. Nevertheless, the study by Rechtschaffen is still one of the most cited works when it comes to the negative consequences of sleep deprivation.
Sleep is essential for mammals, including humans, despite being a potentially vulnerable state. Yet insomnia has long been glorified in modern society. According to the credo, those who achieve a lot, sleep little – a conviction that, according to Peter Stern from the specialist journal “Science”, is slowly changing. “The myth of the strong, successful person who gets by with little time in bed or skimps on sleep to get more done has now been thoroughly debunked,” Stern writes in the introduction to a “Science” feature that explores scientific insights into sleep summarizes.
On average, people spend a third of their lives asleep – and because that occupies such a large part, we take it for granted, according to Stern. The question of its physiological purpose is forgotten. This cannot simply be answered by saying that the body and nervous system need regular breaks.
The brain is highly active during sleep
“We may not be aware of it, but our brains are highly active during sleep,” Stern points out. In recent decades, much has been learned about genes, molecules, cells and circuits involved in sleep and its regulation: “Nevertheless, we still don’t really know why we sleep.”
A clue to the answer to this question could lie in a better understanding of the different stages of sleep. At least that’s what sleep researchers Nicholas Franks and William Wisden from Imperial College London suspect. They mainly relate to the difference between REM and non-REM sleep (NREM): the former is characterized by rapid eye movements and dreams, while the latter mainly describes deep sleep.
It is known from mice that their brain temperature drops by two degrees Celsius during NREM sleep – such a lower temperature could therefore be necessary to set in motion previously unknown regeneration processes in the brain. “Similar to the cleaning crews who come to the empty offices at night and whose work during the day would be almost impossible, when we drift off to sleep and normal brain functions are at least partially suspended, an important and restorative process is underway,” they write .
The REM phase, on the other hand, could represent a kind of test mechanism for the brain to determine whether those regeneration processes were successful. “If that was the case, we wake up,” speculate Franks and Wisden. The scientists also refer to findings on the consequences of lack of sleep.
A study of 8,000 British civil servants found that they were more likely to develop dementia after retirement if they had slept six hours or less for decades. In addition, even 24 hours of forced vigilance impaired driving ability more than drinking the legal maximum amount of alcohol: “It is obvious that our brain function deteriorates when we lack sleep.”
One of those deteriorations could affect our memory. This is suggested by the “Science” contribution by Gabrielle Girardeau from the Institut du Fer-à-Moulin in Paris. Girardeau, who heads a laboratory investigating how sleep strengthens memories, reports an experiment with rats: they first learned the way to a feeding station and were then disturbed while they were sleeping. Specifically, the scientists influenced the so-called Sharp Wave Ripples (SWR). These brainwaves are generated by the hippocampus, a brain region in the temporal lobes, and are believed to be crucial for storing memories. Previous studies suggested that these deep sleep ripples are important for memory consolidation, writes Girardeau.
In any case, the rats in the experiment mentioned forgot how to get their food. However, the researcher also emphasizes that this is only one mechanism in a “multidimensional knowledge space of mechanisms for different memory types, different stages and sub-stages of sleep and the associated physiological patterns”.
removal of metabolic products
In fact, there seems to be not just one, but many reasons why we sleep, emphasizes neuroscientist Laura Lewis from Boston University in another article. Her research with rodents suggests that another function of the night’s rest is to cleanse our brains of harmful metabolites. This is made possible by the interaction of neuronal activity and fluid flow: the electrical currents of the neurons, which are responsible for the consolidation of memories, also cause the tissue fluid to boil, which could have the effect of flushing the brain.
“Sleep maintains the basic physiological health of neurons by clearing their potentially harmful metabolic waste,” Lewis writes. Experiments with mice have shown that in the brain, for example, beta-amyloid molecules are broken down to a much greater extent when they are asleep than when they are awake. At the same time, sleep deprivation increases levels of beta-amyloid in the brains of healthy young adults. An accumulation of these molecules can be found in Alzheimer’s patients, and an increased lack of sleep is also one of the indicators of later Alzheimer’s disease, writes Lewis.
With all these different neurological findings about sleep, however, its fundamental importance should not be forgotten, emphasize Michael Grandner and Fabian-Xosé Fernandez from the University of Arizona in their overview article: “Sleep is a non-negotiable biological state that is essential for maintaining sleep human life is required.”
Our need for sleep is comparable to that for air, food and water – a need that many people around the world only insufficiently meet. In Germany, for example, a third of those surveyed in a 2017 study by the Techniker-Krankenkasse complained about sleep problems, and every second person said they got no more than six hours of sleep. According to the recommendation of the US National Health Foundation, it should actually be seven to nine hours for adults.
The former chairman of the German Society for Sleep Research and Sleep Medicine (DGSM), Alfred Wiater, sees such recommendations from a different perspective. “As sleep researchers, we are often asked why we sleep a third of our lives. No one asks why we can be awake two-thirds of our lives. Sleep and wakefulness have to be thought of together,” says the sleep doctor.
The decisive factor is homeostasis, i.e. the balance within the functional systems of our body. In order for the organism to function in a balanced way in the sense of homeostasis, both the sleeping and the waking state are needed: “And that refers to all regulatory systems, the nervous system, which is more determined by the sympathetic nervous system when awake and by the parasympathetic nervous system during sleep, the endocrine system, the immune system, etc.”
If the homeostasis is permanently disturbed, we would get sick, and how long we sleep is relatively irrelevant. “Our need for sleep is constitutionally determined by our internal clock system anyway,” emphasizes Wiater: “It is crucial that the sleep is restful.”