Written by: Juliann Scholl
Updated March 9, 2021
The discovery of rapid eye movement (REM) sleep (1) in the 1950s sparked an interest in why we sleep and what exactly goes on in the brain while we are sleeping. Sleep science has come a long way since then.
We now understand that sleep plays an important role in memory and learning, muscle repair, and cognitive performance (2). Sleep is a fascinating and intricate process that continues to be studied.
We’ll highlight 15 sleep facts that may surprise you.
1. Scientists Don't Know Why We Sleep
From an evolutionary standpoint, sleeping doesn't seem to make sense. Staying immobile and unresponsive leaves sleepers vulnerable to predators and takes away time from eating and mating.
Although there are vast differences among sleep times for different animals, virtually all animals sleep. Theories on the reasons for sleep include energy conservation (3), keeping safe at night (4), cellular maintenance (5), waste removal (6), and learning and memory formation (4). Nevertheless, scientists have been unable to say for sure why we sleep.
2. Driving While Sleep-Deprived Is Like Driving Drunk
Sleep deprivation affects response times, accuracy, and other cognitive abilities on a level comparable with being intoxicated (7). Going 17 to 19 hours without sleep slows reaction times as much as having a blood alcohol level of 0.05 or above.
Sleep deprivation has serious consequences for road traffic safety. Drowsy driving is responsible for about 800 deaths per year (8) in the United States.
3. The World Record for the Longest Time Without Sleep is 11 Days
In 1964, a 17-year-old student named Randy Gardner claimed the Guinness World Record for the longest time spent awake (9). He had gone 264 hours (11 days) without sleep as part of a science fair project, experiencing extreme tiredness and significant effects on his cognitive and sensory abilities.
Though several people since then claim to have beaten Gardner's record, Guinness no longer recognizes this achievement because of the potential for dangerous health problems.
4. The Amount of Sleep You Need Might Be Genetic
Contrary to popular belief, you can't train yourself to function optimally on less sleep (10). Most adults need between seven and nine hours of sleep to feel their best, and children need even more sleep. Going short on sleep can have long-term health consequences. That said, a small percentage of the population does function well on six hours or less. Studies on twins have found that this might be due to genetics (11).
5. Your Brain Activity Patterns Reveal If You Remember Your Dreams
Virtually everybody dreams, even if they don’t remember their dreams. Although other factors may be at work, a recent study proposed that people who show higher levels of activity in certain parts of the forebrain, notably the temporoparietal junction and the medial prefrontal cortex, are more likely to remember their dreams (12). Waking someone up during rapid eye movement (REM) sleep also has a higher chance of prompting dream recall (13).
6. Your Natural Sleep-Wake Cycle Is Longer Than 24 Hours
Our sleep, appetite, and energy levels rise and fall in synchrony with daylight, a process that's known as the circadian rhythm. Left to its own devices, the human body would maintain a sleep-wake cycle closer to 24 hours and 15 minutes (14). This can be seen in blind individuals who have lost the ability to process light cues. These individuals often have a disorder called non-24-hour sleep-wake disorder, in which the sleep-wake rhythm gets progressively later each day.
7. You Burn Calories While Sleeping
Our metabolism slows while we sleep, but it doesn't stop completely. We still burn calories (15) while lying in bed. We likely burn calories while asleep because brain activity requires energy (16). Still, we burn more calories while awake than asleep.
8. During REM Sleep, Your Muscles Are Paralyzed
As your brain ramps up and starts creating imaginative dream scenery, your body takes the precaution of paralyzing your arm and leg muscles (17) so you don't act out your dreams. Individuals who do act out their dreams are said to suffer from a disorder called REM sleep behavior disorder, which can be dangerous to them and to others around them.
Interestingly, sleepwalking doesn't occur during REM sleep (18), but rather during slow wave sleep, which is not typically associated with vivid dreams.
9. Your Eye Color Might Affect Your Sleep
Being exposed to light in the evening can delay the release of the sleep hormone melatonin, though there is ample evidence that people respond differently to light (19) depending on their age, pupil size, prior light exposure, and other factors. One variable that might play a role is eye color (20).
Researchers in Japan exposed people to light in the evening and measured the effects on melatonin in people with light eyes compared to people with dark eyes. Those with light eyes experienced a more pronounced drop in the sleep hormone when exposed to evening light.
10. You Need Four Nights to Make Up for One Hour of Lost Sleep
The concept of recovery sleep will be familiar to anyone who has ever craved sleeping in on a Saturday after going short on sleep all week. Unfortunately, making up your sleep debt isn't that simple. Although recovery sleep does help, one study found that subjects needed four nights of recovery sleep (21) to compensate for just one hour of sleep lost.
11. Sleep Deprivation Leads to Weight Gain
A night of poor sleep raises levels of ghrelin (22), the hunger hormone, and lowers levels of leptin, the hormone that makes you feel full. As a result, those who don’t get enough sleep may find they have an increased appetite.
There is a strong link between sleep loss and weight gain. Over the long term, sleep deprivation may raise the risk of developing diabetes (23) or heart issues (24), with weight potentially playing a role. The relationship between sleep and eating is bidirectional, as sugar can interfere with sleep quality.
12. People Who Watched Black and White TV Are More Likely to Dream in Greyscale
A 2008 study found that black and white dreams were much more common in older sleepers who had more exposure to black and white television (25). However, another study from the same year found that when asked about their dreams right after waking up, people were more likely to remember their dreams in color (26). This research proposes that color is one of the details from our dreams that we forget after a few minutes.
13. When it Comes to Sleeping, You Can Have Too Much
Although most studies focus on the importance of getting enough sleep, regularly sleeping for more than 11 hours a night has also been linked to health (27) and performance problems (28), including a higher risk of cardiovascular disease (29).
In some cases, oversleeping may indicate an underlying medical condition that needs to be treated. In other cases, oversleeping may be interfering with the circadian rhythm and causing issues. One exception may be athletes, who tend to perform better after getting ten hours of sleep.
14. Cultures Without Artificial Light Split Sleep Into Two Parts
In 2001, sleep scientist Roger Ekirch (30) presented a body of evidence that suggested pre-industrial societies may have split nighttime sleep into two stages (31). In the time they were awake between sleep periods, people would get up to eat and do other activities before returning to bed. This may be validating for people who struggle to obtain consolidated sleep at night, though most sleep experts still recommend getting a solid seven to nine hours of sleep, if possible.
15. Between 5% to 50% of People Sleep with Their Eyes Open
Up to 50% of the population (32) may have a condition called nocturnal lagophthalmos, which prevents the eyelids from closing fully during sleep. Over the long term, this disorder may lead to sleep problems and damage the surface of the eyes (33). If you regularly wake up with dry or tired eyes, ask your bed partner to observe you for a few nights or consult a sleep specialist to see if you might have this condition.
Sleep Is Necessary
Even if some of these sleep facts are surprising, most people recognize the importance of getting a good night's rest. Over the last few decades, it has become more apparent that sleep is essential for regulating stress hormone levels and improving our general wellbeing.
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