Every day almost every person on earth sleeps, yet many questions remain about this state of suspended consciousness and what it actually accomplishes.
We still don't know why we need shut-eye every night, just that we do.
Different myths and explanations have sprung up about all aspects of sleep, from dreams to what it is our bodies do when we're not conscious.
Many aspects of sleep are individual. We differ in the amount of sleep we need and in what can either help us drift off or keep us awake in the late-night hours.
Learning a little more about sleep might help you set up your own tests to see what works and what doesn't.
Here we explore some of the most common myths about sleep in the hope that the answers might lead you to a good night's rest.
Eight hours has long been touted as the gold standard of sleep time, but this number is an average: Most people need seven to nine hours. Some can get by on four hours a night, but others need as many as 12.
"We don't have a lot of data on what exactly the best amount of sleep for a person is, but it's most certainly individual," said Dr. Robert Basner, director of the sleep center at New York-Presbyterian Hospital/Columbia University Medical Center.
While there's a general idea of how much sleep a person needs, the specifics are less clear, and the advice given to determine the adequate amount of sleep can be misleading.
Here's a common suggestion for determining how many hours of sleep you need: How long do you sleep when on vacation, when an alarm clock is not waking you up because of work or other obligations?
But, Basner explained, the number one would get from that is likely to be too high, because we deprive ourselves of sleep beforehand.
"Most people in our society would be making up for lost sleep time," he said.
So trial and error may be the only good method for the average person.
"We don't have any kind of test," said Philip Gehrman, clinical director of the behavioral sleep medicine program at the University of Pennsylvania. "I often say to my patients that if you don't feel fully rested during the day, then you need more."
The majority of people who claim to need next to no sleep seem to have a lack of self-awareness rather than a superior ability to function.
"There's very few people who can consistently get less than four hours of sleep and function during the day," said Gehrman, noting that people who can get by on six or fewer hours are known as "short sleepers."
He noted that when many people who claim not to sleep during the night are observed in a sleep lab, parts of their brain shut down, so although they are no longer fully conscious, they might believe they are.
Basner has made similar observations about people who claim they need or get almost no sleep.
"If someone is functioning well ? and they say they're sleeping one to two hours, they're probably sleeping longer," he said.
Some people can function on very little sleep, but it is unclear whether they'd function better if they allowed themselves to get more.
"Getting by and functioning optimally are two different things," said Basner. "The preponderance of evidence would be anyone sleeping less than three hours is not going to be getting enough sleep and functioning as best as they could."
Instead, the tendency for people who sleep that little is to experience problems down the line.
"Two to four hours would be possible only for a night or two for a healthy person before they were impaired during waking and crash into a deeper sleep the next night," said Rosalind Cartwright, chairman of the department of psychology at Rush University and author of the forthcoming book "The Twenty-Four Hour Mind." "It also reduces glucose tolerance and so makes one vulnerable to diabetes. Short sleep is associated with an increase in obesity, hypertension, diabetes and stroke."
There remains some controversy over whether people need less sleep as they get older. What is clear is that they tend to get less sleep with age.
It's not that people need less sleep as they age, but they have difficulty sustaining sleep for as long as they did when they were younger.
The number of hours of sleep may "decline some in old age only because sleep fragments with more awakenings at night and begins to intrude into catnaps during the day," said Cartwright. "The total hours in 24 is still the same."
While older people may need as much sleep as those who are younger, there is some defined difference when it comes to comparing people at the beginning of life.
"If you take a lifetime perspective, from cradle to grave, this is almost certainly true, as many infants get some 10- to 12-plus hours of sleep a day, and most elderly folks get some five to six hours," said Michael Perlis, director of the Penn Behavioral Sleep Medicine Program in an e-mail to ABCNews.com.
But assuming older people need less sleep because they don't sleep as much as they once did presents problems.
"It's a dangerous myth, because it leads one to dismiss sleep disorders that could be [treated] if they're addressed correctly in older adults," said Dr. Alon Avidan, associate director of the sleep disorders program at UCLA.
While there's no substitute for a good night's sleep, naps may provide a short-term solution to daytime sleepiness if doled out properly.
Avidan explained that power naps of 15 to 20 minutes in the early afternoon (1 to 3 p.m.) are best to help one feel refreshed.
Longer naps, he said, exceeding 45 minutes, can do just the opposite.
"The longer you nap, the more likely you are to wake up from deep sleep," he said, leading you to feel confused and groggy.
Napping too late in the day can lead to insomnia, as it may shift your body's rhythms.
"If you sleep then, the tendency would be to get into the first deep sleep of the night from which you would wake groggy and grouchy," said Cartwright.
The weekends can be (and typically are) used to help catch up on sleep lost during the week, but catching up needs to be done in a specific way to avoid disrupting sleep patterns.
"One of the tenets of good sleep is to stay regular," said Basner.
Using the weekends to sleep in an extra hour or two might be a good way to catch up on lost sleep, assuming one isn't too far behind on sleep to begin with.
The problem, however, is that typically people stay up later on the weekends and then sleep in, which throws off the entire rhythm of the sleep cycle.
One problem, Cartwright said, "is the Sunday night insomnia from oversleeping Sunday morning, so you are trying to fall asleep at too early an hour at night."
Exercise right before bed may keep most people from sleeping, but for others, it might be the answer to their insomnia.
While exercising right before bed might hinder sleep, it is often recommended for earlier in the day, particularly when it exposes one to sunlight in the early morning hours.
"In the broad sense, exercise is generally good for sleep," said Basner, adding that exercise during the day is often recommended for people with insomnia. "People really need to experiment and see what's best for them."
Gehrman explained that for many, exercise can hinder sleep because it elevates body temperature, which remains higher for several hours.
But, he said, he has one patient with insomnia who uses 11 p.m. swims to help him fall asleep.
"Some people deviate from the normal pattern," he said.
Does having sex before bed allow people to destress and fall asleep easier, or does it simply make them more alert and less able to sleep?
"I'm not aware of any studies that have looked at that," said Gehrman, reflecting how a number of sleep researchers responded to this question.
Researchers had a number of explanations for how sex might affect sleep. Sex may help some sleep by calming them down before bedtime. Alternately, it may keep them awake, particularly if it is not enjoyable.
"I'm not sure there's a lot of evidence to answer that well," said Basner. As with exercise, "I guess people have to experiment on their own."
No foods have ever been linked to bad dreams.
But, Basner said, that doesn't mean a large meal at bedtime won't hinder sleep, as it fills up stomach, meaning blood has to work to help digest the food rather than flow to other organs.
"Certainly it's a bad idea," he said.
The primary effect, he and other researchers said, was likely to be reflux and discomfort at bedtime.
"I don't think it's true you're risking nightmares so much, you're just not able to sleep as well," said Basner.
But researchers doubted whether specific foods could bring on bad dreams.
"Only if it gives you a tummyache," said Cartwright.
Warm milk, and turkey, have gained plenty of myth mileage because they contain the amino acid tryptophan.
"[Warm milk] might be relaxing, but here's probably nothing chemically that's going to make you fall asleep," said Gehrman.
While tryptophan may be used as a sleep aid pharmaceutically, its power in food can be severely reduced. Even in pill form, tryptophan is not as strong as sleep aids such as Benadryl.
Avidan noted that the strongest links between foods and sleep come better-known substances, with smoking, alcohol and caffeine all having disruptive effects on sleep patterns.
Myth Based on a Fact
As you may have suspected, the scientific literature on sleep does not show any evidence that Freddy Krueger can kill you by invading your dreams. However, the idea for the film seems to come from a real-life phenomenon linked to a real-life murderous villain.
In the 1970s, millions of Cambodians were killed by Pol Pot's Khmer Rouge regime. Some refugees fled to the United States, and it was observed that some young men were reluctant to sleep -- and some died from heart attacks while sleeping.
Originally attributed to nightmares because of what they'd experienced, the deaths were not restricted to Cambodian refugees. Since that time, evidence has accumulated that sleep deaths resulted from a genetic heart condition called Brugada syndrome.
Rapid-eye movement, or REM, sleep occurs when people are in the deepest sleep, at the time when they dream.
"If people have cardiac disease, they would naturally be more at risk in REM sleep than even being awake," said Basner. "It's not the dream itself, it's the fact that they're in REM sleep."
During REM sleep, the heart rate is less stable than it would normally be, and so it can present a danger to a patient with heart trouble.
"If you have heart disease, REM sleep is a very challenging time," said Basner.
Shortly after the discovery of REM sleep in the 1950s, an episode of "The Twilight Zone," "Perchance to Dream," centered on a man who was afraid to sleep because of dreams of a woman who sought to kill him.
But Basner explained that the fate of the man in the episode (SPOILER ALERT), he suffered a heart attack in his sleep, revealed the futility of trying to avoid sleep to avoid the nightmares, since a sleep-deprived person is likely to rebound into REM sleep once getting to sleep.
"You can't cure that by trying to stay awake," he said, praising the accuracy of the episode.
Heart problems are not the only ones that can emerge when asleep
Avidan explained that some people have difficulty breathing in their sleep and may need pressure on their airways to prevent choking.
In the end, one of the biggest sleep myths may be that sleep is always a benign and restful state.