Humans spend one-third of their life sleeping. The reason behind this has puzzled scientists for a long time. Recently, a new study has shown that the reasons are different when we are young and change as we grow older.
Reasons behind sleeping
There are two leading theories that explain the reason behind sleeping. Both of these focus on the brain. According to one theory, the brain needs sleep to recognize the connection between its cells, build electrical networks that support our memory and our ability to learn. The second theory states that the brain uses sleep time to clean up the metabolic waste that accumulates throughout the day. Neuroscientists have wondered which of these is the primary reason.
In a study published in Science Advances on September 18th 2020, researchers have used a mathematical model that shows that infants spend most of their sleep time in “deep sleep”, which is also known as random eye movement (REM) sleep. During this time, their brains build new connections between cells and grow larger. The brain, however, switches into maintenance mode when children reach the age of two and a half. REM sleep time falls drastically and the brain uses sleep time for cleaning and repairing mostly.
Senior author Van Savage, who is a professor of ecology and evolutionary biology and of computational medicine at the University of California, Los Angeles, and the Santa Fe Institute stated that it was very shocking for them that this transition from growth to maintenance mode is so sharp. The researchers collected data in other mammals – rabbits, rats, and guinea pigs. They found that their sleep pattern undergoes a similar transformation, but it is too soon to tell whether these patterns can be seen in other species as well.
Leila Tarokh, who is a neuroscientist and Group Leader at the University Hospital of Child and Adolescent Psychiatry and Psychotherapy at the University of Bern has a different point of view. Even though she was not involved in the study, she says that the transition may not actually be that sharp.
According to her, the development pace varies between individuals, and that they had sparse data points between the age group of 2 and 3. She added that if the researchers had studied individuals as they aged, they might understand that the transition is not so sudden but more smooth, or that the age of transition varies between individuals.
A new hypothesis
In a study published in the Journal Proceedings of the National Academy of Sciences, Geoffrey West, Savage, theoretical physicists found that an animal’s brain size and brain metabolic rate can precisely determine the amount of time the animal sleeps, more so than the animal’s body size. Generally speaking, big animals with a big brain and low metabolic rate sleep less than a small animal with opposite features.
This rule stands correct across various species and between members of the same species. For example, mice sleep more than elephants and newborn babies sleep more than adult humans. With the knowledge that sleep time is less when brain size is bigger, the authors wondered how quickly this change occurs in different animals, and if this relates to the function of sleep over time.
Tarokh noted that babies sleep about twice as much as grownups, and spend a larger amount of their sleep time in REM, but the question still remains as to what function that serves.
In their study, the authors found that the metabolic rate of the brain is high during infancy. During this time, the organ builds many new connections between cells, which in turn results in more time in REM sleep. They reached the conclusion that long hours of REM sleep during infancy support rapid remodelling in the brain, as new networks develop and babies pick up new skills. Savage noted that between the ages of 2 and 3, the connections don’t change as quickly, and thus the amount of time spent in REM decreases.
During this time, the metabolic rate of cells in the cerebral cortex also changes. In infancy, the metabolic rate is proportional to the number of existing connections between brain cells and the energy needed to develop new connections in the network. As the rate of construction slows down, the relative metabolic rate also slows down.
Development psychologists refer to this early period of development as a “critical period” of neuroplasticity. Neuroplasticity is the ability of the brain to form new connections between its cells. Tarokh pointed out that after the critical period, the plasticity doesn’t go away, but it slows down significantly. At the same time, the proportion of non-REM sleep increases, supporting the idea that non-REM is more important to brain maintenance than forming new connections.
Going forward, the authors plan to apply their mathematical model to other animals as well, to see if a similar pattern exists.
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