Why Do We Sleep?

People spend about a third of their lives sleeping, and scientists have long debated why sleep takes up so much of our time. Now a new study shows that our main reason for sleep started as one thing, then surprisingly changed at a certain age.

Two leading theories as to why we sleep focus on our brains: The first theory says that the brain uses sleep to reorganize connections between its cells, setting up electrical networks that support our memory and ability to learn. The other theory says that the brain needs time to clear the metabolic wastes accumulated during the day. Neuroscientists have debated which of these functions is the main cause of sleep, but a new study reveals that the answer may be different for infants and adults.

In the study, published Sept. 18 in the journal Science Advances, researchers think babies spend most of their sleep hours in “deep sleep,” known as REM sleep. Again, researchers think, just before toddlers reach the age of 2 and a half, their brains go into maintenance mode, using sleep time mostly for cleaning and repair, significantly reducing the amount of REM sleep.

“It was absolutely shocking to us that the transition from growth mode to maintenance mode was so sharp,” said senior author Van Savage, professor of ecology and evolutionary biology at the University of California, Los Angeles and Santa Fe. The researchers also collected data from other mammals (i.e. rabbits, rats, and other guinea pigs) and found that their sleep could undergo a similar transformation; however, it is too early to say whether these patterns are consistent across many species.

A Prominent Hypothesis

In a previous study published in 2007 in the journal PNAS, Savage and theoretical physicist Geoffrey West found that measuring an animal’s brain size and brain metabolic rate accurately predicted the amount of time the animal slept. In general, large animals with large brains and low brain metabolism sleep less than smaller animals with contrasting characteristics.

This rule applies between different species and between members of the same species; for example, mice sleep more than elephants and newborn babies sleep more than adult humans. However, knowing that sleep duration decreases as the brain grows, the authors wondered how quickly this change occurred in different animals and whether it was related to sleep function.

To begin answering these questions, the researchers pooled the available data on how much people sleep, from newborn babies and children up to age 15; They compiled data such as body size, metabolic rate, and the ratio of time spent in REM sleep to non-REM sleep at different ages.

The research workers created a mathematical model to track all these changing data points over time and see what differences emerged between them. They found that the brain’s metabolic rate was high during infancy when the organ was making many new connections between cells, which was associated with more time spent in REM sleep. They concluded that as new neural networks form and babies acquire new skills, the long REM hours in infancy promote rapid brain remodeling.

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