By the time you’ve lived the average Canadian lifespan of 82 years, you will have spent about 27 of them asleep.

That’s a third of your life. Eight extra hours every day you could have spent earning more money, making the world a better place, travelling, mastering a hobby, or simply enjoying being alive. To some, it might seem like a terrible waste.

David Samson has little sympathy. The associate professor of anthropology at U of T Mississauga points out that the South American owl monkey spends an astonishing 17 hours asleep every day, while even chimpanzees, our closest relatives, sleep away almost half of each day. “Just be happy you’re not an owl monkey!” he says.

Anyone who has ever missed a night or two of sleep knows how important it is. We need sleep to think clearly and stay healthy. If we miss too much, we have a harder time concentrating and regulating our emotions. Our physical performance declines, and we are more likely to get sick. Go without sleep long enough and, eventually, we die.

But if sleep is so important, why do humans – by many measures the most intelligent species on the planet – need so little of it? “The paradox, and the mystery, is that we are the shortest sleeping primate of any ever recorded,” Samson says.

In his new book The Sleepless Ape, Samson argues that our unusually short sleep isn’t simply an evolutionary curiosity. It may have helped shape what it means to be human. By sleeping more efficiently than our primate relatives, he suggests, early humans gained time to strengthen social bonds, learn new skills and ultimately expand across the globe.

Out of the trees

The story started 1.8 million years ago, when our ancestors left the safety of the trees to live full-time on the ground. (We’re still the only primate that routinely sleeps there.) Life on the ground left them far more vulnerable to being attacked, so the pressure was on to shorten the amount of time they spent asleep. “Sleep is vulnerable, and it’s costly. All things being equal, there’s always something else to do. If you can get away with less of it, evolution is always going to try and reduce it,” Samson says.

Over millennia, our ancestors adapted in a few ways. First, they became even more social. Living in larger groups meant it was more likely someone would be awake to act as a sentinel.

To this day, people living in subsistence societies are surprisingly wakeful. In research with the Hadza people of Tanzania, Samson found that individuals had staggered sleep schedules, and that there was almost always at least one adult awake at any time.

People also learned how to build shelters and control fire, allowing them to keep out predators and pests and regulate temperature and exposure to light, creating better conditions for sleep. The idea is that because they were secure and comfortable, they could sleep more deeply, in uninterrupted blocks of time. This sleep was far more restorative than the insecure, fragmented sleep that came before. “We’re getting this high-quality sleep, and we’re doing it in the shortest amount of time possible,” Samson says.

Our way of sleeping led to many benefits. For one thing, it gave us more time to build social bonds, find mates, seek and prepare food and invent new technologies. “It’s a feedback loop. The increase in sociality led to securing sleep sites, and then those secured sleep sites encouraged more activity, such as shoring up alliances or learning new skills,” Samson says.

In fact, the deeper, more restorative sleep our paleolithic ancestors were experiencing might have improved our memory and learning capacities, further intensifying the feedback loop.

Once humans began using fire to extend the day, our relatively short sleep times enabled us to take full advantage of the extra light. “No species on the planet can use technology to manipulate light at night, except for humans. That opened a completely new social niche and new temporal niche, which we could access, so we could take advantage of this reduced sleep time,” Samson says. The extra learning and sociality set the stage for our expansion out of Africa and into the rest of the world, he notes.

OK. But why sleep at all?

Samson’s theory addresses one mystery: why humans sleep so little. But scientists still face an even bigger question – why do we sleep at all?

The fact that we spend a third of our lives disengaged from the world, vulnerable and unproductive suggests sleep must serve an important purpose. But surprisingly, scientists aren’t exactly sure what that is. “There’s lots of conjecture, but nothing definitive,” says John Peever, a professor of cell and systems biology who studies sleep and neurological disorders. “What sleep likely does is provide some form of restoration for the body and the brain. But the real question is, what exactly is being restored, and how,” he says.

Even the simplest living things have periods of activity and rest, regulated by circadian rhythms. These internal clocks likely evolved with the first cellular life as a way of keeping organic processes synchronized with each other and with the Earth’s light and dark cycle.

But true sleep seems to have evolved much later, probably 200 to 300 million years ago in animals. Sleep is more than just rest. Its defining features are physical inactivity, a decrease in response to the environment, and characteristic changes in electrical activity in the brain.

All animals that we know of sleep. But the costs to the animal are potentially high, making them vulnerable to predators. As Allan Rechtschaffen, a pioneer in the field of sleep research, proposed, “If sleep does not serve an absolute vital function, then it is the biggest mistake the evolutionary process ever made.”

The question is, what is that vital function? We know that an awful lot happens while we sleep. Cells are repaired, waste is cleared from the brain, the immune system is strengthened and memories are consolidated. But it’s not clear which, if any, of these is the “core” function of sleep.

Richard Horner is a U of T professor of physiology and medicine. He points out that once an organism has evolved to sleep, that becomes a convenient time to do all sorts of routine maintenance that doesn’t necessarily require sleep to occur. “A lot of things happen during the night because eight hours of rest is as good a time as any to do jobs x, y and z,” he says.

Part of the puzzle is the way our sleep is structured, with two basic types: REM (rapid eye movement) sleep and non-REM sleep. In non-REM sleep, heart and respiration rates decrease, the muscles relax and the brain waves slow. It is the deepest stage of sleep. In REM sleep, the eyes move rapidly back and forth under the eyelids, and muscles in the body twitch. It’s when vivid dreaming occurs. Peever says that slow-wave, non-REM sleep seems to have a restorative function. In fact, inducing slow waves in the brain while people are awake is restorative all by itself, according to recent research.

The function of REM sleep isn’t as clear, although it could help us process and consolidate memories. “There’s this very interesting idea that REM sleep is taking the information that we experienced while we were awake and then reinforcing it by replaying it,” Peever says.

Birds do it, bees do it

One reason sleep remains so mysterious is because of the wide variation in sleep patterns among animals. For instance, elephants only need four hours a day, while armadillos need 20. A mother whale can go a whole month without sleep right after giving birth. The amount of REM sleep animals need also varies significantly. So, if it serves a fundamental purpose, why do different species do it in such radically different ways? If it were essential for cognitive function, Peever says, you might expect an animal as intelligent as an elephant to need more of it.

Common antidepressants suppress REM sleep in humans. But millions of people experiencing reduced REM sleep don’t seem to be having any problem learning new things, Peever says. “REM sleep is really one of the biggest biological mysteries.”

Nevertheless, Horner says, we do understand the big picture of how sleep works, even if many details remain unresolved. “Sleep is a process of rewiring the brain to make an organism better at being awake the next day,” he says.

Animals need to process experience and learn from it. For reasons we don’t fully understand, sleep appears to be essential to that process – despite requiring animals to become less aware of the outside world.

During sleep, Horner says, experience is likely replayed – in the process moving from temporary storage in a brain region called the hippocampus to permanent storage in the cortex.

The other processes that happen during sleep, such as cell repair and maintenance, are secondary, Horner believes. Its primary function, he argues, is to build and adapt the brain’s neural connections in response to our experiences.

The paradox, the mystery, is that we are the shortest sleeping primate of any ever recorded

Modern sleep

People in Western industrialized countries tend to complain that they don’t get as much sleep as they need. The common assumption is that artificial light, modern life and too much screen time keep us awake too late and get us up too early. In fact, compared with subsistence societies, we’re doing pretty well, Samson says. They average about seven hours of sleep a day – a little less than the eight hours most of us think we need.

Samson thinks our real problem might be with circadian function – that is, matching our internal clocks to our sleep-wake cycle. Because we spend so much time indoors, we don’t have the benefit of exposure to sunlight that would normally keep our circadian rhythms strong and coordinated. “We know objectively that sleep in small-scale subsistence societies is shorter and more fragmented. But they have higher circadian function. We can get the best of both worlds if we double down on spending more time out­side, being mindful of our light cues,” Samson says.

For anyone rushing through modern life, eight hours of sleep can feel like time stolen from everything else. Viewed through the lens of evolution, however, humans have struck a bargain. We sleep relatively little – yet somehow manage to do everything sleep seems to require.

If Samson is right, those “lost” hours aren’t simply the price of being human. They may be one of the reasons we became human in the first place.