Why do we sleep? Scientists try to answer the question

Updated: 11:42 PM EDT Jun 4, 2022

It’s strange when you think about it: Human beings spend nearly a third of their lives lying down with their eyes closed, dead to the world. But we have a biological need for sleep because it delivers numerous unbeatable benefits; it gives us more energy, reduces stress, and bolsters the immune system. While we sleep, our bodies also repair cells and fine-tune hormone levels.Related video above: Americans Want Better Sleep! Here Are Some Ways They’re Trying to Get That Done“Sleep affects almost every tissue in our bodies,” Dr. Michael Twery, a sleep expert, writes in a blog post for the National Institutes of Health. “It affects … our immune system, appetite, breathing, blood pressure, and cardiovascular health.” On the flipside, not sleeping enough can have disastrous consequences on our wellbeing. Drowsy people are much more likely to make poor decisions and be involved in traffic accidents. And chronic sleep deprivation can lead to a weakened immune system and increase your risk of developing depression.These are all compelling reasons to hit the sack, but they don’t completely explain why humans spend 8 hours unconscious and paralyzed each night. As noted sleep scientist Allan Rechtschaffen once said, “if sleep doesn’t serve some vital function, it is the biggest mistake evolution ever made.” For decades, scientists have been trying to solve the mystery of why humans sleep, with four possible theories paving the way: the inactivity theory, the energy conservation theory, the restorative theory, and the brain plasticity theory. Inactivity TheorySometimes referred to as the “adaptive” or “evolutionary” theory, the inactivity theory was one of scientists’ first stabs at an explanation behind sleep, made back in the 1920s, before scientists had started sleep labs or discovered rapid eye movement. The idea is that for animals, night is a vulnerable time, when darkness makes it difficult to move safely or avoid predators. Lying still and silent was a good way to avoid danger until morning. Through evolution, the theory posits, this strategy eventually morphed into what we now call sleep. The inactivity theory has one glaring problem, though: falling unconscious at night would have heightened vulnerability and made it nearly impossible to react to danger. Energy Conservation Theory This theory suggests that the primary function of sleep is to cut down on the amount of energy an animal needs and how much it uses. For most of us today, grabbing a snack is generally a simple task, but for ancient humans, finding food required a ton of time and effort. Sleeping through the night—a time when hunting was challenging and hazardous anyway—was a good strategy to conserve energy.This theory is backed up by research that shows when humans are asleep, their metabolism slows by approximately 10% (this figure is higher in other species). For example, both body temperature and caloric demands drop while we sleep. Many researchers consider the energy restoration theory to be part and parcel of the inactivity theory.Restorative TheoryAfter a long night of sleep, we often feel not just rested, but restored. Some scientists think that physical and cognitive restoration is actually why we sleep. The restorative theory has gained steam in recent years thanks to a slew of convincing animal and human studies. For instance, in Rechtschaffen’s experiments, carried out in the late 1980s, lab animals deprived entirely of sleep lost all immune function and died in a matter of weeks. Scientists have also discovered that most restorative functions—including muscle and tissue repair, protein synthesis, and the release of growth hormones—occur while humans sleep.Additionally, it appears sleep somehow boosts our immune system. A 2009 Carnegie Mellon University study showed that people who got seven or fewer hours of sleep each night were about three times more likely to become ill when exposed to the common cold than those who got 8 hours or more. But it’s not just your body that’s rejuvenated while you catch your Z’s. When you are awake, a chemical called adenosine accumulates in your brain. A byproduct of cellular activity, it’s adenosine that is thought to make us feel tired; as more builds up during the day, the more tired we become. During sleep, the body clears adenosine from the brain, allowing us to feel refreshed and alert when the alarm goes off.Brain Plasticity TheoryOne of the newest and most exciting theories about why we sleep is based on research that suggests sleep is correlated to changes in the structure and organization of the brain, a phenomenon known as brain plasticity. This theory, sometimes called the information consolidation theory, forwards the idea that during sleep, our brain sorts through the information we have acquired that day, throwing out data we don’t need and storing the rest in long-term memories. Several studies support this notion, demonstrating that sleep deprivation has a negative impact on our ability to learn and recall information.Scientists still aren’t exactly sure how sleep and memories are connected, though many researchers think that the brainwaves during different stages of sleep may be correlated with certain types of memories, such as memories that preserve facts and others that preserve procedures. The brain plasticity theory may explain why infants and young children, whose brains are still developing, need so much sleep. Infants, for example, spend up to 16 hours a day asleep, and much of that time is spent in REM sleep, the stage during which most dreams occur.Whether sleep transforms our brains or conserves precious calories, scientists are unlikely to discover one, indisputable reason behind sleep. In fact—while these theories may have distinct names and proponents—the reason why we sleep is likely a combination of these ideas, or quite simply, all of the above.

Humans were not the only ones who wanted 2020 to be over faster. The Earth spun faster than normal last year, according to scientists.

Last year, the Earth broke the previous record for shortest astronomical day, set in 2005. In fact, the Earth broke the record 28 times in 2020. And it's still spinning faster.

While Earth is, on average, pretty reliable and takes 86,400 seconds to rotate around its axis, it’s not perfect.

"When highly accurate atomic clocks were developed in the 1960s, they showed that the length of a mean solar day can vary by milliseconds (1 millisecond equals 0.001 seconds),” write Graham Jones and Konstantin Bikos on TimeandDate.com.

How fast or slow the Earth spins can be impacted by various factors.

"Changes in the atmosphere, specifically atmospheric pressure around the world, and the motions of the winds that may be related to such climate signals as El Niño are strong enough that their effect is observed in the Earth’s rotation signal," David A. Salstein, an atmospheric scientist from Atmospheric and Environmental Research, said in 2003.

These variations can add up. LiveScience reports the Earth has actually been slowing down for the last several decades. When the time it takes earth to make a full rotation deviates from the Coordinated Universal Time (UTC) by more than .4 seconds, the UTC is adjusted.

Scientists have been adding a “leap second” every year-and-a-half on average. The last one was added on December 31, 2016.

Since the Earth has sped up, scientists believe each astronomical day in 2021 will be 0.05 milliseconds shorter, and over the course of the year, it adds up to a 19 millisecond difference.

"It's quite possible that a negative leap second will be needed if the Earth's rotation rate increases further, but it's too early to say if this is likely to happen," physicist Peter Whibberley of the National Physics Laboratory in the U.K., told The Telegraph. "There are also international discussions taking place about the future of leap seconds, and it's also possible that the need for a negative leap second might push the decision towards ending leap seconds for good."

The average person may not notice a leap second being taken away or the ones that have been added, but they will impact things like navigation, spaceflight, computer networks and astronomers.

The last time every day of a calendar year was shorter than 86,400 seconds was in 1937, according to TimeandDate.com.

The Biden administration has pledged to "address the climate crisis at home and abroad."

While some of the details have yet to be released, wildlife conservationists say it's a step in the right direction.

Scientists say polar bears, koalas, elephants, and butterflies are just some of the species directly impacted by climate change.

“We have huge drivers of rapid environmental change like climate change, and it’s impacting wildlife species," said Dr. Megan Owen, Senior Director of Wildlife Conservation Science at the San Diego Zoo Wildlife Alliance. "And wild places and the very planet we all share across the globe it’s a huge problem."

And while she says it's a "huge" problem, she also says it's a chance for humans to make a difference.

“I can tell you that in the most straightforward way that we have to reduce our greenhouse gas emissions, there’s no alternative," said Dr. Owen. "The faster we do it, the bet. It’s a commitment to being a part of a solution rather than continuing to behave as we always have, knowing that we’re degrading our own habitat in that process.”

The evidence, Dr. Owen said, is there.

She uses polar bears as an example.

“I don’t think any other species illustrates as well the real tangible impact of climate change the polar bear sea ice habitat is literally melting away.”

Butterflies are experiencing a 2% decline in population as temperatures are warmer and warmer for more extended periods.

“People don’t necessarily notice a catastrophic decline in the case of butterflies, for example," Dr. Owen said.

And then there are koalas, who we all quite literally watched disappear as fires raged through Australia in 2019 and 2020, killing thousands of koalas.

As for elephants, they are living with drought just like people.

“There are more wells dug for people because they need water to survive as well, and the wells that get dug are traps for baby elephants and are in no way intentional," Dr. Owen said.

As for what we can do about it, Dr. Owen said we should support regulatory action to support green energy policies.

She also added that we should support people in local governments, state governments, national governments that are climate forward and think about how we can move our energy sources into greener pastures.

Paul Baribault is President and CE of the San Diego Zoo Wildlife Alliance, which operates the zoo, safari park, and 200 conservation scientists that support work at home and abroad.

“We have to think about collectively, as humanity, how do we best manage that balance?" Baribault said. "How do we keep wildlife in balance with our own balance and maintain our healthy ecosystem maintain healthy wildlife because that will have a direct result on our own health.”

The Biden administration announced that the U.S. would target reducing emissions by half by 2030.

Baribault says making that effort is a good step for the future co-existence of both animals and people.

"If we get everyone to focus on their own community and small things that they can do, simple things, like turning off lights, not running water while brushing teeth,” says Baribault.

It's little things that turn into big things for the greater good.