Why do we still not understand why humans need to sleep?

By Tré LaRosa
NeuLine Health

Just about any problem in biology can be reduced to two fundamental questions related to natural selection and evolution: How does this process affect our ability to survive and how does this process affect our ability to reproduce? For some problems, such as trying to understand why bats might have developed the ability to fly, or why male birds are more colorful than females, or why humans walk on two legs, these questions related to survival and reproduction relatively easily frame the answers. For the question of sleep, these fundamental questions are still, well, fundamental, but the most basic explanations aren’t as clear. Thousands of years ago, sleep would’ve posed a significant risk for our long-ago ancestors by reducing us to defenselessness while unconscious; sleep also doesn’t seem to influence sexual selection and reproductive success. Since sleep is a common trait of so many species, scientists are able to study the potential reasons sleep evolved in other species with the hope learning those reasons might help to tease out the origins and reasons for why humans need to sleep. Is sleep an example of convergent evolution, that is, did different species independently evolve the need for sleep? Are the reasons for sleep different for different species?

If you take a few minutes to research sleep, you’ll find most sources mentioning the many “reasons” we supposedly need to sleep; these range from memory upkeep (both consolidation and removal of memories), early brain development and growth, neuroprotection and neural development (deep sleep may help protect against Alzheimer’s), some researchers even believe why we sleep and dream is to creatively solve real-world problems. But none of these reasons or theories posit an explanation for the underlying biological reason for what forces our bodies to sleep.

For something we spend approximately a third of our lives doing, our understanding of sleep is surprisingly lacking. The brain, in its infinite complexity, is the chief organ responsible for sleep regulation, so maybe the lack of understanding around sleep isn’t that surprising. It might not seem like knowing the origins for why humans first developed the need to sleep would influence our treatment and approach to sleep but this isn’t exactly true. If we can develop an understanding of these biological underpinnings, then we can better understand how genetics, environment, diet, and everything else influence our sleep patterns and sleep disorders.

In this wonderful write-up by The Atlantic on the state-of-the-art pioneering research institute International Institute for Integrative Sleep Medicine, a fascinating question is posed that gets to the heart of the issue of why we don’t quite understand why sleep has evolutionarily developed across the animal kingdom: “What is the physical substrate of sleepiness?”

If you’re unfamiliar, substrates are the biological substances on which enzymes act. One of the simplest examples of substrate-enzyme interactions is carbohydrase and carbohydrates; that is, carbohydrase is the enzyme that breaks down the carbohydrates we eat, so in this case, carbohydrates are the substrates. Comparatively, we don’t know what is building up (or being broken down) while we sleep that directly explains the physiological and biological need to sleep. This is the simplest way to articulate substrate-enzyme interactions.

Scientists are trying to solve this question from many different angles; some are investigating humans who have sleep disorders such as narcolepsy and insomnia to understand why some people spontaneously fall asleep whereas others have an inability to doze off with the hope that both issues are related to this currently unknown substrate; other researchers are studying other animals, such as mice which can be connected to EEGs, by physically preventing them from falling asleep and measuring the biochemistry and signals produced by their brains produced while in this sleep-deprived state, then allowing them to fall asleep and measuring the same signals to compare. Even more strangely, we know that animals in a hibernation state are in a physiologically different state than when they are slumbering.

If you’re reading this, I think there is a pretty good chance that you have had days where you feel you are too tired to function. I say this because in a recent study, 72% of people reported feeling sleepy multiple times a week. You’ve probably had days where you’re in the middle of a tedious task on a computer and your eyelids slowly droop a bit or where you can’t stop yourself from yawning (and pouring an extra cup of coffee in the afternoon); this is your body not telling you it desires sleep, but rather, that it needs sleep. This is what is called homeostatic sleep pressure, or homeostatic sleep drive. A similar homeostatic process you might be familiar with is hunger drive, which is not that unlike sleep drive except that we have a better understanding of why we need to eat. The graphic below, from the CDC, helps to visualize this process, whereby we accrue sleep pressure, then alleviate that pressure by sleeping through the night, only to accrue pressure the subsequent day