An average person will spend more than 20 years of their lives sleeping. Why?

Anyone who has ever watched professional acrobats in a circus, jugglers on the street, ice skaters, pianists or cello players surely must have been surprised with the extent to which people can develop various abilities by constantly practising. But few have thought of developing their ability to participate more actively in their own dreams during the night. Psychologists from the University of Adelaide in Australia have decided to try just that: to develop techniques that can increase their chances of having so-called “lucid dreams,” in which dreams are very intense and the person is fully aware of the dream and has an impression of control over that experience.

The first technique is to develop the habit of testing reality. A person should acquire the habit of checking their surroundings as many times a day as possible and deciding whether they are asleep or awake. This should assist the brain to continue to do this when the person is asleep. Another technique is intentionally waking up after five hours of sleep and remaining awake for a few minutes. Then, a continuation of sleep is recommended, coupled with preparation for probable lucid dreams in the so-called REM phase that should follow. The third technique is the so-called MILD technique (from “Mnemonic Induction of Lucid Dreams”), where a person also wakes up after five hours of sleep and then repeats the phrase “the next time I fall asleep, I will remember exactly what I dream about”, imagining what the dream will be.

Combining these three techniques has led to the achievement of the desired effect in almost every fifth person. They would experience lucid dreams in which they actively participated. However, the MILD technique alone has led to lucid dreams in nearly half of the people who did manage to fall back to sleep within just a few minutes. This technique is based on something commonly referred to as prospective memorisation, i.e. the ability to remember obligations that will need to be performed in the future. Repeating the sentence “next time I fall asleep, I’m going to remember exactly what I dream about” forms an intention. When the brain enters a state of lucid sleep, it feels obligated to remember everything that is happening and to participate actively in it.

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In their work, scientists are increasingly reaching towards the concept of so-called “citizen science”, where doing some of the components of the research process is left to people who aren’t actually scientists. The diversity of their experiences and ideas may solve problems in which scientists tend to get stuck. I recently saw just how potentially useful this concept could be on my Facebook wall.

I shared the post in which I stated that it was a pity that we had to shorten our already relatively short lives by sleeping. Why our organism has to “switch off” regularly, and what it actually does during that time, is still one of the greatest unknowns of modern science. There are many theories about it, from consolidating new memories and storing them for a long time, to recovering and putting the immune system in order, while removing everything harmful from the body. But the only thing that seems certain is that all of the more complex living beings on Earth, from the smallest fruit fly to the largest blue whale, have to sleep to live. In terms of evolution and survival as a species, sleep is very dangerous — leaving oneself at the mercy of predators, and without the possibility of feeding or mating. When thought of it this way, it’s rather questionable how has the habit of sleep survived at all in nature.

I also shared information with my Facebook followers that scientists from Caltech University showed that the species of Cassiopea jellyfish — which has no central nervous system, but only nervous branches — also needs some kind of sleep at night. Based on this observation, they concluded that sleep is evolutionarily older than the brain itself. Such unusual breakthroughs can be the trigger for a whole new approach to thinking about a problem, and sleep is quite a puzzle indeed. That information suddenly opened up a space for possible new hypotheses. During the night and morning, a spontaneous discussion developed on my Facebook wall. One of my friends suggested that sleep may have originated even before living organisms began to evolve. It is, therefore, possible that we actually live to sleep, during which time we may be performing some important part of our earthly mission. He offered the possibility that sleep may be older than evolution itself, and could have arisen as early as the first life.

And others, then, went on to add their own hypotheses: how sleep may well be our real existence, and what seems like reality may just be a strictly controlled hallucination. Others have suggested that in our dreams, we may cross into parallel universes, where we also have lives. Another friend suggested that we may all just be in a video game with limited memory. If that was true, then every sleep may have a function that we all get “loaded” with a new level for the next day. Following up on that thought, another friend speculated that maybe it was the other way around, and we were really all in some “Matrix”, and during sleep, the information we collected during the day was entered into the “cloud”.

Some have caught onto the fact that exposure to the “hormone of happiness and love” — oxytocin — reduces the need for sleep, which may mean that oxytocin and sleep actually match in some sense. One of my friends hypothesized that sleep could be a universal quest for happiness. Others pointed that without enough sleep a life cannot be sustained, but without a state of wakefulness it could potentially protract if the steady supply of energy is ensured — such as to those in a long-term coma. Therefore, it is sleep, not wakefulness, that should be a suitable condition for long-distance transport through space. This led to the idea that perhaps the first living beings arrived to Earth from somewhere in space while sleeping. If this was the case, then sight and wakefulness would develop only later on, under the influence of sunlight.

Indeed, why do jellyfish also have cycles of day and night, as if the light was important to them? Another interesting question is why have only some animals developed night vision, and not all of them? It was pitch black on the planet on cloudy nights before the invention of electricity, and night vision would give an incredible advantage to any predator. Another idea from the community on Facebook was that it was perhaps too boring for everyone at night, so all the living beings simply went into an “energy-saving state” until the sun rose again. This raises further questions because sleep has currently been shortened in humans due to artificial light, smartphones, tablets and laptops, which is all making us behave contrary to the way we evolved to sleep. Someone made a comment that dolphins only sleep with half of their brains, alternately, so that they can actually always be awake. Human babies, on the other hand, sleep randomly, with little regard for whether it is day or night, and depending on when they’re fed, and only later on do they adopt daily rhythms.

If I spent weeks thinking about sleep, I would hardly manage to come up with the number of interesting hypotheses on this subject that my Facebook friends developed in just one single night, through the application of a “citizen science”. Some even wondered what happens with a salamander that lives in the dark waters, while others then remembered that speleologists who stay in caves for a long time often start sleeping in very unusual rhythms. It is a little bit unnerving to realise that we spend almost a third of our lives asleep, i.e., typically more than twenty years of our entire existence, and we still understand so little about why this happens.

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What, in fact, do we know about sleep? Historically, various civilisations have compared sleep to death. In Greek mythology, the sleeping god Hypnos and the god of death Thanatos were both children of the Goddess of the Night, Nyx. In modern times, however, a whole branch of medicine has developed that deals exclusively with sleep, its disorders and its connection with human health and diseases. It is generally accepted that sleep is a natural state of mind and body that is necessary for life and therefore constantly repeated. During sleep, a person’s consciousness is altered. The activity of the senses is absent, all of the muscles under our voluntarily influence are still and we don’t have the ability to communicate with our environment. Sleepers can not respond to stimuli, but unlike a coma, waking up from normal sleep is much easier.

When we’re asleep, most of the biochemical mechanisms in our bodies are switched to anabolism, i.e., general metabolic biosynthesis. It helps restore the immune, nervous, skeletal and muscular systems to the best of their function for the next day. These are vital processes: they are crucial for maintaining mood, memory, thinking, normal hormonal balance and a quality immune response. Our internal, so-called “circadian rhythm”, which is maintained within the cells, promotes sleep at night. However, people can also suffer from various sleep disorders, including insomnia, excessive sleep, irregular breathing during sleep, sleepwalking, and disturbances of the aforementioned circadian rhythms. It is known that sleep problems can eventually lead to psychiatric disorders, such as depression and bipolar disorder, and also to alcoholism. As many as nine in ten adults who suffer from depression will also experience some sleeping difficulty.

Scientific research on sleep, however, has identified at least some clues that could help us understand the role of sleep better. When we sleep, the brain uses significantly less energy than it does during our waking state. This is especially the case in one of the two characteristic phases of sleep — so-called “non-REM” sleep. The brain is filled with adenosine triphosphate (ATP), a molecule used to store and transfer energy. When we’re awake, the brain is responsible for as much as a fifth of the body’s total energy expenditure, which is a large share of the total. It’s interesting how, when we’re asleep, the threshold of the senses rises, especially our hearing, so we are able to ignore the information that continues to arrive. However, we will still react and wake up to very loud sounds or dramatic flashes.

Furthermore, changes in the secretion of various hormones occur during sleep, with largely uncertain consequences. An area of the brain called the suprachiasmatic nucleus is thought to be related to our “internal clock”. This part has a clear neural connection to the pineal gland, which releases the hormone melatonin during the night. Growth hormone is secreted in waves only during certain phases of sleep, while prolactin is secreted both during the day and night, especially in women. Levels of cortisol, a stress-related hormone, are mostly increased throughout the night, culminating upon waking up, while they then become decreased during the day.

During sleep, two very different phases alternate. One phase, the so-called “non-REM’’ phase, which has been mentioned earlier, is considered to be “deep sleep”. At this stage, our body temperature and heart rate drop, our brain uses less energy, and we probably have very little awareness of ourselves. However, this phase is interrupted during sleep by interesting shorter periods, the so-called “REM” sleep phase (from the English “rapid eye movement” — which characterise this, considerably more active phase). During REM sleep, also known as “paradoxical sleep,” we can expect awareness and dreams, but also nightmares. The electrical potential waves read by the electroencephalogram (EEG) are quite different in these two phases of sleep. Each REM-non-REM cycle lasts for about an hour and a half. During each night, this cycle can be repeated between four and six times. Awakening usually occurs either during the REM phase or after its completion. During awakening, the EEG reads increased electrical activity in the brain, which begins in the thalamus and then spreads further to the cerebral cortex.

Today’s science is convinced that the brain has a particular need for sleep for its recovery among organs. In the rest of the body, the processes of recovery can take place during dormancy even in the waking state. It is thought that the brain removes waste products from the metabolism during sleep much faster than it can when awake. A lack of sleep certainly has a negative effect on the efficiency of the immune system. Conversely, it has also been recognised that the activation of the immune system during the body’s immune defence processes against infectious diseases affects sleep.

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In modern times, human sleep is changing significantly when compared to all previous historic periods. It is not entirely clear what consequences this could have for the health at the population level. In this day and age, many people wake up at a specific time, with the help of an alarm, and sleep differently on weekdays and weekends. All of this can lead to disturbances in their internal circadian rhythms, i.e. the proper alteration of wakefulness and sleep.

Circadian rhythms are especially interesting because they are maintained and adjusted by cycles of light and darkness. However, people isolated from the natural light and left for a long time in conditions where there is no light at all, such as in underground caves, or where the light is constantly on — such as in a submarine — will still continue to sleep. Their daily cycles, related to their metabolism and hormone secretion, will also resume, but the duration of these cycles is likely to become slightly longer than 24 hours. However, re-exposure to natural light will return that inner, circadian clock to a 24-hour rhythm. During these daily cycles, human body temperature will also oscillate between 36°C and 37°C.

Our internal clocks are very sensitive to light. If we’re exposed to light during the night, it can suppress melatonin secretion and increase our body temperature and alertness. At the same time, blue light has the strongest effect, leading to increasingly common fears that the use of electronic devices before bedtime may interfere with our normal sleep. In modern times, people often find themselves in situations where their internal clock is confused and disturbed . This could be due to night shifts at work, air travel to different time zones, and interior lighting inside their homes. Prior to these major changes, during most of the year, a young and healthy adult would fall asleep a few hours after sunset, experience a minimum body temperature at around 6 a.m., and wake up a few hours after sunrise.

The cultural differences between nations and civilisations in their sleeping habits can also be quite interesting. Some sleep in one piece, while others sleep two or more times during the day, as a part of their culture. When, in underground experiments, people are isolated from daylight, they begin to alternate more often between being asleep and awake within 24 hours. So, they tend to sleep several times during the day unless they are entertained by something sufficiently exciting. Before the Industrial Revolution, many societies generally had habits of sleeping during the night but also in the afternoon. With the industrial revolution and urbanization, people adopted the model of sleeping in one piece — the so-called “single-phase” sleep.

Particularly interesting experiments related to the “normal” human inner rhythm of sleep were conducted by the famous French speleologist Michel Siffre. This adventurer, scientist and explorer of subterranean caves graduated from the famous Sorbonne University. Among other things, in 1962 he founded the French Institute of Speleology. He spent several months inside underground caves like Scarasson, or caves in Texas, with no idea of ​​what the weather was like on the surface. He organised similar expeditions for other speleologists. Based on his research, he noted that several speleologists, quite surprisingly, developed 48-hour sleep cycles, instead of 24-hour ones. The results of his experiments were also used by NASA because several astronauts also confirmed Siffre’s conclusions. One particularly interesting observation was how human isolation from understanding external time can also lead to short-term memory loss. That inner, so-called circadian rhythm is clearly an interesting topic. For their contribution to understanding the molecular mechanisms of this process in living cells, American scientists Jeffrey Hall, Michael Rosbash, and Michael Young were awarded the 2017 Nobel Prize in Physiology and Medicine.

In addition to the cultural habits of entire nations, individuals can also greatly differ in their sleeping habits — from the length of sleep, where some need only four hours a day, and others as many as 12 hours, to when they like to fall asleep. This divides the so-called “early risers” from the “night owls”. This habit of sleeping with respect to the time of day is known as the personal chronotype. Genes are likely to have a significant impact on the chronotype, as monozygotic twins, with identical genetic guidance, have significantly more similar sleeping habits than dizygotic ones.

Perhaps the most interesting feature of sleep is the dreams themselves — experiences in which we feel like we exist and meet known and unknown people. Dreams may have elements that would be utterly impossible in our waking reality. Sigmund Freud assumed that dreams were a symbolic expression of frustrations and desires that were repressed into the unconscious part of the mind. He used the interpretation of dreams in his psychoanalysis, trying to bring forward these repressed desires.

Today, one of the most interesting theories on the origin of dreams was proposed by scientists John Allan Hobson and Robert McCarley. They suggested that dreams are caused by the random “triggering” of neurons in the cerebral cortex during the REM phase of sleep. The front of the brain then begins to create a story, trying to reconcile this random and meaningless sensory information presented to it. So, dreams would arise as the frontal brain tries to connect it all into a story that seems grounded in some familiar context. This theory has gained popularity because it also helps explain the apparent irrationality of the mind during the REM sleep period, as well as why dreams can have such unusual stories.


Declaration: Professor Igor Rudan, FRSE, is the President of the International Society of Global Health; co-Editor-in-Chief of the “Journal of Global Health”; Joint Director of the Centre for Global Health and the WHO Collaborating Centre at the University of Edinburgh, UK.

TWITTER: @ProfIgorRudan

FACEBOOK: Professor Igor Rudan



Translation to English credit: Lauren Simmonds

Image credit: Valeria Andresson,

Director of the Centre for Global Health and the WHO Collaborating Centre at the University of Edinburgh, UK; Editor-in-Chief, Journal of Global Health

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