"When humans come down from the trees, that's when we begin to sleep deeply."

BarcelonaIt is said that in current times there is a A veritable "epidemic" of sleep problemsScreens, stress, anxiety... Falling asleep has become a true act of resistance. However, sleep has been an essential act in the evolution ofHomo sapiens Since the dawn of humanity, only during these hours of deep rest can we repair the brain, consolidate memory, or reorganize our emotional circuits.

Juan Antonio Madrid, professor of physiology and founder of the Chronobiology and Sleep Laboratory at the University of Murcia, knows a great deal about all of this. In his latest book, The dream of the sapiens (Editorial Platform, 2025), invites us to reconnect with one of the most natural, and least valued, acts of our species.

One of the questions the book answers is How can sleep be indispensable for our survival?When we sleep, we are completely vulnerable and unprotected. This situation wouldn't have been very advantageous for our prehistoric ancestors. "Despite being more vulnerable to attack, the immobility of the body and the darkness of night also provided protection," explains Madrid. Not to mention that a prehistoric hunter-gatherer would have been much more susceptible to attack if they didn't sleep, because they would have lost many reflexes and motor coordination.

Sleep is essential for our species and has been practiced since the beginning of time. According to studies conducted on tribal groups that still live in a way very similar to our ancestors, such as those found in the Kalahari Desert in Africa, it seems we slept directly on the ground, in groups and outdoors, protected by the warmth of fire. "The moment we transitioned from living in trees to walking, sleeping, and living on the ground, fire became a fundamental element," explains Madrid. Until then, sleeping in a tree like chimpanzees, with the inherent instability, disrupted many important parts of sleep, such as REM sleep. "The risk of falling prevented us from entering these sleep stages, but when we descended to the ground, we began to sleep more deeply," he points out. And fire played a role in all of this: "It defended us from predators, allowed us to see in the dark, gave us warmth, and protected us from parasites and mosquitoes," the author continues.

Every day, our bodies are programmed to signal us to sleep and gradually shut down at night thanks to substances like melatonin. Similarly, as sunrise approaches, our bodies also increase glucose and cortisol levels to wake us up. And in between, we spend a few hours sleeping, a process that still fascinates experts today. Sleep is known to have three phases: a first phase in which sleep is less deep; REM sleep; which is when we have dreamsAnd a third phase in which we have the deepest and most restorative sleep, which is what restores our bodies. However, during these hours of immobility, we always have a kind of secretary, the thalamus, which acts as a filter, waking us up in case of emergency. A very specific example is that of mothers who wake up at any sound their baby makes, but, on the other hand, don't wake up if they hear noises from the street.

Different life stages

Children, adolescents, adults, and the elderly don't all have the same type of sleep. "Our bodies are genetically programmed to make changes throughout our development, and sleep is no exception," Madrid points out. From the third trimester of pregnancy and during the first years of life, many hours of sleep are needed, primarily during REM sleep, the stage of dreaming. "This is probably because this phase serves to connect different areas of the brain and promotes neuronal connectivity, two very important things in children's neurodevelopment," she explains.

Later comes adolescence, a time when a sleep revolution begins, as sleep had previously been of good quality and without too many problems. "They fall asleep later and later and wake up later in the morning. Their biological clock changes, which directly clashes with having to teach first thing in the morning," Madrid points out. Another key moment is old age, when sleep hours are greatly reduced and sleep is less deep and dreams are less frequent. "But sleep also becomes more fragmented, and during the day they compensate with short naps," she adds.

According to Madrid, there are now more than eighty sleep disorders, a number that has increased since the invention of electric light and the beginning of the Industrial Revolution. This marked a shift from relying on solar time to creating schedules based on the possibilities offered by artificial light.

Therefore, the expert recommends avoiding screens until we get out of bed or right before going to sleep. "Ideally, as soon as we get out of bed, we should be exposed to natural daylight; and three hours before going to sleep, we should be in a warm light environment, not the white or blue light of screens," he concludes.