What is the Definition of Sleep?


There’s a lot we don’t know about sleep. We still don’t know, for instance, exactly why we sleep.

“Sleep…is playing a vital function in our and other species’ survival–even if we don’t know for sure what that function is at this stage,” Nadine Gravett, a neuroscientist studying sleep at University of the Witwatersrand in South Africa, told QZ.

We also don’t know whether our ancestors slept through the night or polyphasically.

But here’s a little of what we do know about sleep.

Clinically defined, sleep is the body’s rest cycle and is triggered by a complex group of hormones and neurotransmitters that respond to cues from the body and its environment. Sleep is a period of rest for the body and brain where consciousness is partially to fully suspended. It is associated with lower movement levels, but easily reversible awareness to stimulus from the environment. Physicians often will time when sleep begins, ends, and transitions through stages based upon the specific signals, measured in electrical waves, that the brain produces at a given time. The stages of sleep are explained more in later sections, but it’s interesting to note that there’s no one time that sleep begins and ends, exactly. The first stage of sleep is known as a grey area between wakefulness and sleep, while level 2 is definitively in sleep.

What happens during sleep

When you take in new information your brain fires a spark between your neurons called a synapse. Recent research on mice showed the sleeping brain saves strong and vital synapses and prunes weak ones. Researchers theorize that your brain saves memories that matter, like material that you expect to be on a test, or a conversation with a loved one while you sleep and discards less-important memories to save the caloric energy and space it takes to maintain them.

Sleep is also critical for learning and memory. Not only does sleep help to sustain attention and concentration to learn new things, but memories from the previous day are consolidated and stored in long-term memory during sleep. Some studies also show during sleep you can learn new things. One study showed that people associated good and bad odors with sounds played while they were asleep.

Sleep is also when your brain takes out the trash, so to speak. Just like you, every cell in your body takes in nutrients and poops out waste. The cell food comes in via your blood vessels, and the poop goes out via lymphatic vessels -- except in your brain, whose nooks and crannies are too tight. A recent study in rats showed their brains shrinking up to 60% during sleep so a separate union of slimmer janitors called cerebrospinal fluid can flow through.

These guys pick up toxic molecules such as beta-amyloid, which may contribute to neurodegenerative disorders like Alzheimer’s and Parkinson’s, and throw them out.

But it’s not just your brain that’s hard at work (sort of) while you’re asleep. While you’re out, your body sends blood from your brain to your muscles and secrets human growth hormone for healing and repair.

More sleep leads to better immune health and function. Cytokines produced when you are sick are primarily produced when you sleep. This is thought to be the reason why infectious illnesses like the flu causes lethargic reactions in the body. Rest gives the body time to produce these proteins and to restore itself for wellness.

Why sleep matters

Sleep is very important to a healthy lifestyle. It offers the body a chance to recover from wear and tear of daily life. Many researchers have proven the restorative effects of sleep. We know the body repairs itself during certain sleep stages. During sleep, cells regenerate themselves while temperature, heart rate, and breathing drop to conserve energy for this vital process. We may not know exactly why we need to sleep, but we do know what happens to our brains and bodies when we don’t sleep enough.

Sleep deprivation decreases:

Insufficient sleep can decrease your average reaction times as much as alcohol. And it may even make you feel pain more acutely.

And the damage to your health builds over time. A researcher at Uppsala University in Sweden showed that sleep deprivation increases your likelihood of developing obesity. Not sleeping enough also raises your risk for developing certain psychiatric disorders including anxiety, depression, dementia, and Alzheimer's disease. It can also contribute to PTSD and suicide.

But as bad as not getting enough sleep is for your health and performance, getting enough sleep is a huge boon. A recent survey of 200 high-achievers showed that sleep was the most important measure of optimized mental performance. The New York Times lists a bevy of benefits, saying sleep “can measurably improve your memory, overall cognitive performance, ability to learn new information, receptivity to facial cues, mood, ability to handle problems, metabolism, risk for heart disease and immune system.” Getting enough sleep may help people read emotions, process external stimuli, and respond better to stress. It may also boost your memory.

Is anyone sleeping enough?

The CDC has officially declared sleep deprivation a public health epidemic. A third of Americans qualify as chronically underslept, and 80 percent of people experience sleep issues at least once per week. One 2016 study showed sleep loss costs the US more than $400 billion each year and every year US workers miss 1.23 million days of work due to sleep deficits.

Between 50-70 million Americans suffer from the more than 70 different types of sleep disorders that exist, the most common of which is insomnia, or difficulty falling and/or staying asleep.

Most people need between seven and nine hours of sleep per night. Some of us need even more. Most of us aren’t getting enough, and don’t realize it.

When it comes to how much sleep you need, “There’s a real wide variation,” says Dr. Andrew Lim, Neurologist at Sunnybrook Health Sciences Centre. How much sleep is ideal for you depends on your gender, lifestyle, and age. “There are some people who really function at their neurological and physiological peak on six hours of sleep. And there are other people who really do need like eight, nine hours of sleep to feel fine. The easy answer is: About seven and a half hours. The more correct answer is: It depends.”

Who needs more sleep?

The evidence is strong that at least three categories of people probably need more sleep than average: young people, women, and athletes.

Young people’s sleep needs

A comprehensive literature review of 320 articles by the US National Sleep Foundation found that infants can require up to 17 hours of sleep per day. Kids need at least 10 hours per night, while teens needs eight to 10. The National Sleep Foundation recommends that adults get between seven and nine hours of sleep per night for optimal health, performance, and longevity.

Women’s sleep needs

Because our brains are more active during the day, some researchers believe that women need more sleep than men. Women are better and more prolific multitaskers than men, and multitasking burns more energy than single tasking. All that extra energy expenditure during the day means women need about 20 more minutes of sleep per night than men.

More sleep also improves women’s sex lives. In one study partnered women who got an extra hour of sleep were 14 percent more likely to engage in sexual activity the next day and reported higher levels of sexual desire and women who slept longer on average reported more vaginal lubrication during sex than women who slept shorter. A Fitbit study showed that the average woman gets 24 more minutes of sleep than the average man.

Athletes’ sleep needs

Another category of human that tends to need more sleep: Athletes. One Stanford study looked at how sleeping for 10 hours per night for around 7 weeks impacted football players’ performance and found it improved by more than 50%.

For example, a detailed sleep plan is central to LeBron James’ seven-figure body-care regimen. His trainer Mike Mancias told Tim Ferriss that getting between eight and 10 hours of sleep per night, is the key to his “never-ending” recovery.

Many athletes, according to Michael S. Jaffee, Vice Chair of Department of Neurology at University of Florida, are experimenting with sleeping longer than normal to enhance their performance. And many pro sports teams have hired sleep consultants to improve their athletes’ sleep.

Dr. Charles Samuels is a sleep physician and the Medical Director at the Centre for Sleep and Human Performance where he runs the Sleep Medicine Program. “Endurance athletes require a tremendous amount of sleep,” Dr. Samuels told me. “Certainly beyond the eight-to-nine to recover adequately based on what they do. And when you think of what endurance sports are now, compared to when I was a kid, they’re extreme.”

Dr. Samuels trained with an Iceland Fire and Ice Ultra winner and Dave Proctor, who attempted to break the cross-Canada running record by running 100 kilometers per day for 66 days straight. “Dave Proctor will tell you sleep played a massive role in his ability to go across, and he never really focused on sleep in his entire professional career as an ultrarunner,” Dr. Samuels said. “He just never even paid attention to it. We believe people like that need tremendous amounts of sleep for recovery.”

No one is getting enough sleep

Despite all the advantages to getting enough sleep, QZ reports that the average night’s sleep has been getting shorter over time. Before the Industrial Revolution, some estimate Americans got two more hours of sleep than we do today. Today we get one hour less than the 1940s, when most American adults were averaging 7.9 hours. Fitbit and the National Sleep Foundation both found that American adults average 6.9 hours of sleep per night.

Between 1942 and 2013, the percentage of Americans who regularly got the recommended seven to nine hours of sleep per night dropped from 84 percent to 59 percent. Of course in 1942 no one had Netflix and a radio cost like $300 (in today’s dollars). Yet this rose-colored view of sleeps past might be false nostalgia. A recent study found that modern-day members of three different hunter-gatherer societies get even less sleep than many Americans.

Currently about half of people worldwide get less than six hours a night, according to the Guardian. More than a third of Americans typically get less than seven hours of sleep, and a similar proportion of Brits get less than six hours per night.

Public health authorities blame nightly screen time (whether it’s smartphones, tablets, computers, and televisions) for disrupting our natural circadian rhythms and the hectic pace of modern life for creating a sleep deprivation epidemic.

Sneaky sleep deprivation

One reason people binge watch Lost instead of sleep is that they assume that the amount of sleep they need is the amount that makes them feel rested. Even New York Times health author Tim Herrera toes this line, writing, “Generally, if you’re waking up tired, you’re not getting enough [sleep].”

In one study, people who had been awake for 36 hours got more confident in their answers to tests as time went on, and were most confident about their wrong answers. As people age, they become less likely to underestimate how much sleep deprivation impairs their cognition.

Going by how you feel is tricky because sleep deprivation actually decreases your self-awareness, especially as it relates to your cognitive performance. In other words, not getting enough sleep will impair your thinking while convincing you that your thinking is fine. Like alcohol, sleep deprivation simultaneously impairs performance and increases confidence.

Season 2 of The User’s Guide to Cheating Death begins with an episode on sleep. Dr. Colleen Carney, Associate Professor and Director at Ryerson University tells host Tim Caulfield that some people delude themselves into thinking that they’ve trained themselves to cope with the physical and mental effects of sleep loss. These people believe, “that they’ve trained their bodies to not develop the diseases that they’re going to develop,” Dr. Carney says. “And we know that that’s false. The same diseases are going to befall them but they’re going to delude themselves into thinking they’re doing alright while it’s happening.”

How to find out how much sleep you need

We know getting enough sleep is essential for optimal performance and health. But exactly how much is enough for you? After all, there’s a pretty big gulf between seven and nine hours.

One way to find out how much sleep is enough for you is to take a break from your alarm clock. If your work schedule is flexi you can do this anytime. If not, find a time when you don’t have to be anywhere in the morning for two weeks.

Once you begin, go to bed at the same time every night and allow yourself to wake up naturally. Ignore the first few days when your body will likely be catching up on lost sleep. After that, if you are consistent on when you go to sleep and don’t let anything external wake you up you should start sleeping the same amount of time each night. By the end of the two weeks you should have a good idea about how much sleep your body needs.

How to measure your sleep duration and quality

To find out whether you are sleep deprived, you want to combine objective and subjective data to get a complete picture. First, you’ll want to get a sleep tracker. It’s true that multiple studies have shown that sleep trackers are less accurate than polysomnography (the gold standard for sleep studies). They consistently overestimate sleep efficiency and total sleep time.

Learn more: The Truth About Sleep Trackers

However, sleep trackers are still useful for measuring changes in your sleep patterns.

If you start using an advanced sleep-tracking device today you’ll be able to see whether your lifestyle changes impact your sleep duration and quality.

Sleep trackers work best when combined with a sleep journal.

Here’s an example of a sleep journal:

sleep journal.png

A sleep journal helps you find out how much sleep is optimal for you and what sleep and wakeup times work best for you.

Some people operate better when they go to bed and get up early, while others are natural night owls. Writing down when you went to bed and got up and how you felt the next day helps you see your patterns.

When combined with a sleep journal, advanced sleep tracking can provide insights into how different kinds of sleep impact your mental and physical performance.

If you’d like to start tracking your sleep with a sleep tracking device plus an automated sleep journal, download the Biomarker app here.

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