The short answer: adults need 6 to 9 hours per night. Around 7 to 7.5 hours of actual sleep (not counting time falling asleep and getting out bed) appears to be optimal for most people.
The long answer: it depends. The amount of sleep each person needs depends on many factors, including age, health, recent physical exertion, and mental activity. There is genetic influence, too. Some people just need more sleep than others and this runs in families.
Infants sleep about 16 hours a day, while teenagers need 9 hours on average. For most adults, 7 to 8 hours a night appears to be the best amount of sleep, although some people may need as few as 5 hours or as many as 10 hours of sleep each day.
Don’t assume you are at one end of the spectrum unless you have paid close attention to your body. If you are drowsy during the day, even during boring periods, you haven’t had enough sleep the previous night. Most people experience a dip in early afternoon – siesta time. But if you fall asleep in the afternoons consistently, it means you haven't had enough sleep at night.
Sleep deprivation – even one or two nights – can vastly affect your need for sleep. Unlike many things in life, sleep time is not something that is routinely changed. You can’t get used to a lower amount of sleep just because it fits your schedule. If you try to, it will affect your judgment and reaction time, even if you are not consciously aware of it. The reasons for sleep – we still don’t know. But you can’t resist it for long. Sleep deficit can be cured only by getting some sleep.
If you routinely fall asleep within 5 minutes of lying down, you probably have sleep deprivation or a sleep disorder. Microsleeps, or very brief episodes of sleep in an otherwise awake person, are another mark of sleep deprivation. In many cases, people are not aware that they are experiencing microsleeps. Some suspect that the widespread practice of "burning the candle at both ends" among harried workaholics has changed perceptions so much that what is really abnormal sleepiness is now considered normal.
It is known that both long sleepers and short sleepers have higher mortality rates than people who sleep around the standard 8 hours. What should the public health recommendations be on amount of sleep?
To support the National Healthy Sleep Awareness Project (NHSAP), the U.S. government’s Centers for Disease Control and Prevention commissioned the American Academy of Sleep Medicine and the Sleep Research Society to develop a recommendation for use in public health discussions. After a survey of literature and experts they concluded adults should sleep 7 hours or more per night to say healthy. They also concluded that it is not clear whether long sleep (over 9 hours per night causes a health risk.
A Gallup poll (2005) of Americans past age 50 found only 32% reported getting a good night's sleep routinely. 56% said they got between 6 and 8 hours a night. The US Dept of Health and Human Services reports that “The odds of being a short sleeper (defined as someone who sleeps less than 6 hours a night) in the United States have increased significantly over the past 30 years”
Sleeping past normal wakeup time can lead to post-sleep inertia or worn-out syndrome. This is similar to jetlag: the body sleeping sleeps at odd times. It leads to lethargy and a cloudy mind.
Do people even know how much sleep they are getting? No. When falling asleep the memory function can be shortchanged. The mesograde - middle time length - amnesia of sleep makes people overestimate sleep latency.
Sometimes you’ll hear that you need less sleep as you get older. But that is incorrect. Seniors often sleep less than young adults and children because they have insomnia. Also, deep sleep (stage 3) in many elderly people declines to a lower percentage of total sleep time, and may even stop completely. People subjectively experience the decline in stage 3 sleep time as a decline in sleep quality. Objective tests on healthy people show that depriving them of deep sleep negatively affects cognitive skills.
Is this newfound insomnia normal? It’s hard to say for any individual person. Some experts consider insomnia a normal part of aging, or it may result from medical problems that are common in elderly people and from the medications and other treatments for those problems. A recent Dutch study found that older people who slept a lot had higher cholesterol levels and lower good high-density cholesterol than those who slept less.
"Eight hours they give to sleep" – Sir Thomas Moore - Utopia, 1551
The federal government's Healthy People initiative has established a goal of getting more people to get adequate sleep on a regular basis. Their metric of sufficient sleep is 8 hours for people 18 to 21 and 7 hours per night for adults over 21. According to their numbers, 69.6% of the population meets this goal, and the government wants to raise this to 70.9% by 2020.
Our proposal: The master sleeper.
Sleep is fun, but we can have fun while awake, too. Here's another reason there is no fixed amount of sleep you need:
You're sleepy. What makes the sleepiness disappear? Sleep, of course. But what else? Increased wakefulness. A more challenging and interesting wake life works against sleep, too.
Finding average sleep times is not straightforward, because individuals are notoriously inaccurate. You can’t necessarily believe what people report about how much they slept last night. Tests with actigraphy have found that in general people overestimate how much they sleep.