Scientists Still Don't Know Why We Need to Sleep

In spite of the fact that we spend more than one third of our life sleeping, science has not been able to explain yet why we need to close our eyes at night (or during the day, depending on jobs and preferences). While some may argue that sleeping is essential for relaxing the organism and recharging one's batteries for the following day, this is disproved by behaviors seen in migrating whales, dolphins and birds, which swim and fly, respectively, for days on end without needing to stop for sleep. The fact is that, hundreds of studies into the issue later, no one really knows why we need the time-out.

From a strictly evolutionary standpoint, sleeping is not exactly the most appropriate thing an animal can do in its environment. During sleep, it is more open to attacks, cannot mate, scout for food, engage in social behavior or other such actions, which it normally does. “It's been thought that sleep must serve some as-yet unidentified physiological or neural function that can't be accomplished when animals are awake,” University of California in Los Angeles (UCLA) sleep researcher Jerome Siegel attempts to provide an explanation.

“So you have to start thinking – if sleep has a vital universal function, how are they able to survive without it?” Siegel asks, speaking about migrating animals. Counter-intuitively, they are not able to go on without sleep while engaged in normal activities, but can do so under immense stress and high energy requirements, which come with flying thousands of miles across the planet in one go. If a human tried that, he or she would undoubtedly die of exhaustion and/or dehydration a couple of days later, experts say, quoted by LiveScience.

However, there are animals at the opposite pole. For instance, Siegel mentions, the big brown bat spends roughly 20 hours of its day in slumber. “One might suppose this animal sleeps so much because of some unknown function that sleep has for it. But it seems more easily explained by the fact that it preys on moths and mosquitoes that only come out at dusk and are active for only a few hours before the temperature falls and they can't function. If it spent more time awake, it would spend more energy but not be as successful at hunting, and if it came out in the day it would be exposed to predatory birds that can see much better than it can,” the expert explains.

“All species have times when they need to be active and ones where they don't, and so you can see species optimizing their periods of inactivity in response to their environment. I'm saying that sleep helps animals adapt to the world around them,” Siegel adds.

Among the most circulated theories that attempt to explain the role of sleep, experts include one that holds sleeping as being invaluable for storing memories accumulated throughout the day and sorting out the irrelevant ones, one which states that sleep reverts oxidation stress that occurred during the day, and a third that holds that sleeping promotes longevity. But Siegel disagrees with most of them.

“If one knows the size or brain size of different animals you cannot predict or explain their sleep time or depth. The baboon has the same total sleep time and total REM sleep time as the guinea pig. The long-living elephant has one of the shorted [sic.] sleep times whereas the long-living bat has one of the longest sleep times,” he says.