How Much Sleep Do Adults Need?
How Much Sleep Do Adults Need?
Each one of us has a unique sleep requirement. Our sleep need depends upon genetic and physiological factors and also varies by age, sex, and previous sleep amounts. However, a simple definition of sufficient sleep is a sleep duration that is followed by a spontaneous awakening and leaves one feeling refreshed and alert for the day.
Sleep Function and Need
Need for sleep is complex because it is bound up with the more general question of the function of sleep. As we do not understand the exact function of sleep, and it is possible that sleep serves many purposes, simple benchmarks for defining adequate sleep are difficult to identify. Normal individuals perceive that sleep is restorative. We know that deprivation of sleep makes us sleepy and results in poor performance while sufficient sleep improves our alertness, mood, and performance. Sleep may also provide significant long-term health benefits, but there may be many modifying factors such as the age of the individual, duration of sleep and influence of co-existing health problems and life-style and environmental factors. Determining the specific amount of sleep that is sufficient for optimal performance of a task is difficult since it may vary depending upon the task that is being performed, the time of day the task is performed, and the level of performance desired. To explore the relationship between sleep duration and important outcomes, most research has examined correlations between various sleep durations and levels of performance.
The relationship between sleep duration, performance and health is important and timely. Between 1959(1) and 1992(2) the average amount of sleep reported by middle age individuals decreased by about one hour per night (from 8-9 hours per night to 7-8 hours per night). A study examining the sleep duration from time diaries (records of sleep time and awake time) of full time workers from 1975 to 2006(3), found a significant increase in the number of individuals who were sleeping less than 6 hours per night. A recent study from the National Health Interview Survey which examined the sleep duration of individuals across several occupations ranging from manufacturing to public administration found that the percent of workers who reported a sleep duration of 6 hours or less per night increased from 24 to 30%(4) in the last 20 years. These findings probably demonstrate the development of widespread partial sleep deprivation or sleep "restriction" which is most likely related to external environmental or social factor(s) such as the need to work more than one job or longer work shifts rather than a biologic change in need for sleep. The important question is the extent to which such changes produce negative consequences for performance, health, and/or quality of life.
Much research investigating sleep duration requirements has examined reduced sleep duration because, as evidenced above, chronic or long-standing sleep restriction is increasingly pervasive in the community. Studies of short sleep duration have shown that this "restricted" sleep can be associated with increased sleepiness, poor performance, and increased health risks or mortality.
One way that researchers examine the effect of restricted sleep time, is by having individuals with a given (presumably "normal") sleep duration, usually 7 - 8 hours, sleep for shorter intervals, such as 2 - 7 hours for one or more nights. A common type of research examines changes in performance of specific tasks following normal sleep duration and compares it to performance after a period of sleep restriction. Another type of research employs health surveys or questionnaires that are given to many individuals and which ask about sleep duration and other health characteristics and outcomes. Associations between these features and naturally occurring differences in sleep duration across the study participants are determined by established statistical methods.
Investigators have measured various aspects of performance after sleep duration has been decreased from 8 hours to 7 hours or less for one or more nights. For example, one study showed that participants were significantly more sleepy on the day following reduction of time in bed from 8 to 6 hours (5). Other studies have shown increased sleepiness and delayed response time (similar to taking longer to apply the brakes when trying to stop a car) when sleep was restricted to 5 or 6 hours per night (6), (7), (8) for several nights. Notably, participants who are allowed to sleep for only 6 hours per night for 12 nights responded as slowly as other participants did after one night without any sleep at all (8). Additional changes associated with similar total sleep loss include decreased short-term memory, poor performance on newly learned or complex tasks, and difficulty maintaining attention (9). In addition, individuals report increased sleepiness and decreased positive mood after sleep restricted to five hours per night (10). It is of further concern that, although the time required to fall asleep decreases and the quality of performance deteriorates as the number of nights of sleep restriction increases, an individual's perception or subjective assessment of his or her sleepiness starts to level off after a few days. Thus, individuals may develop some tolerance to feelings of sleepiness over a few days, and this may make it more likely that sleep restricted people will be unaware of their continuing deterioration in alertness and performance (8). This can have profound personal and public safety consequences (e.g. safe motor vehicle operation, ability to make critical work and family decisions, etc.).
The studies suggest that the degree to which sleep duration is commonly restricted in the community may compromise alertness and performance. However, because it becomes a chronic condition, sleep loss may be unrecognized by the sufferers who accept it as their norm. Some clues to an overly sleep restricted life include need for stimulants like coffee to wake up or get going each morning, difficulty remaining focused and productive when sitting for a while, negative mood, or poor memory.
Sleep Extension or Prolonged Sleep Duration
While it is clear that short sleep duration engenders a variety of adverse health and social consequences, few studies have examined whether longer than usual sleep times improve performance or alertness. One investigation sought to encourage college students to "sleep as much as possible" over several weeks (11). The study participants increased their total daily sleep time from 7.5 hours to 9 - 9.9 hours for the first week. However, by the end of the study (one to 7 weeks later), total daily sleep time decreased to about 8.5 hours. This implies that the students made up for previous chronic partial sleep deprivation on initial nights and approached their underlying maximum ability for normal sleep by the end of the experiment. Compared with measures made prior to the research-related increased time in bed, the increased sleep times were associated with improved subjective alertness and longer times required to fall asleep on daytime nap tests (such nap tests are an objective way of measuring sleepiness such that the less sleepy and individual is, the longer it will take him/her to fall asleep during the day when asked to do so). There was also a significant improvement in reaction (response) time, although, this finding could be due to practicing on the task during the experiment. The results of this research study suggest that college students can increase their sleep time but probably cannot make themselves into permanent long sleepers. Another study, analogous to the sleep restriction studies described earlier, included a condition where time in bed was increased from 8 to 9 hours. This increase did not result in significant changes in performance (12). To our knowledge, only one study has shown reduced performance after extended sleep (13) while several have shown improved alertness and mood, particularly in individuals who may have been chronically partially sleep deprived prior to the studies. The studies generally support the ability of normal young adults to sleep an hour or longer each night with slight improvement in alertness and performance.
Sleep Duration and Health Consequences
Several research studies have shown that sleep restriction to about 4 hours per night on 1 - 2 nights has significant effects on normal individuals. Studies have shown an increased heart rate and blood pressure(14), increased inflammation as measured by C-reactive protein (a marker of inflammation which can be measured in the blood and which has been proposed as a risk factor for coronary artery disease -(15), impaired glucose tolerance (which can be a prelude to the development of diabetes-(16), and increased hunger/appetite (which could promote obesity -(17) In addition, information obtained from questionnaires in large sample groups has also shown statistical associations between chronically reduced sleep duration and increased risk of hypertension (particularly in women) (18), (19); diabetes(20); and weight gain(21), (22), (23). These results are particularly meaningful because they agree with the results of studies that examined these issues by experimentally reducing the participants' time in bed. There is also a suggestion that sleep restriction may adversely affect immune function. In one study, young adults who had been given a flu shot after four nights of reduced sleep had less than half of the antibody response 10 days later compared with individuals having normal sleep at the time of vaccination (24). Changes in these clinical markers after only a few nights of partial sleep loss typically reverse when normal sleep is allowed. However, it is important to recognize that many of the above studies examined the effects of relatively short-term alteration in sleep duration but many individuals in the community may have chronic partial sleep deprivation. The consequences and reversibility of any consequences in this regard are not known.
The results of studies done to date imply that numerous health consequences could be associated with sleep restriction. In contrast, increased sleep duration has not been associated with immediate negative health consequences. However, controlled long-term studies remain to be done.
The relationship between sleep duration and lifespan has been most frequently been examined as part of large health screens given to thousands (or millions) of individuals. Respondents are typically asked, "How many hours do you usually sleep each night?" and the association between the responses and mortality at a later date is evaluated. Two such recent large studies and a summary of 23 such studies from the past 30 years have shown that, compared with individuals with a 7-8 hour sleep duration, there is an increased risk of dying in individuals who reported a short sleep duration (usually substantially less than 7 hours) and in individuals who have reported a long sleep duration (generally 9 hours or more) (25), (26), (27). Two studies have also analyzed the responses to a second questionnaire about six years later and a follow up 17 - 22 years later to determine who had died and the relationship to sleep duration. People who consistently reported short and long sleep on both questionnaires still had increased mortality (26), (27). In both studies, individuals who initially slept for about 7-8 hours but had begun sleeping either less or more than this by the time they had completed the second questionnaire had an increase in mortality(27), (26). Of interest, individuals who increased their sleep length from short at the first observation to average at the second observation no longer had increased mortality at the end of the study period. Finally, individuals who decreased their sleep duration from long to 7-8 hours no longer had increased mortality (26).
As discussed earlier, numerous experimental studies have shown associations between short sleep durations and markers for diabetes, obesity, and hypertension. The presence of these common conditions is also associated with mortality. However, long sleep durations have not been associated with these medical problems (28). In a recent study that replicated the classic finding of increased risk of death for both short and long sleepers, the data were analyzed to consider the effect of age; and the increased risk associated with both short and long sleep durations disappeared in the younger individuals (ages 32 – 59) but not in older individuals (ages 60 - 86) (29). Further, it was found that the prevalence of both long and short sleep durations increased greatly in 70 and 80-year-old individuals (plotted in Figure 1). This suggests that changes in sleep duration were common starting only a few years before death and might reflect inflammatory processes or undiagnosed underlying medical or psychiatric problems in low socioeconomic individuals. For example, long sleep was strongly related to a history of depression or antidepressant or anti-anxiety medication use; a report of living alone or having fewer children; or the lack of employment or low social economic status (30). This implies that long sleep durations reported in surveys may be difficult to differentiate from simply spending a long time in bed each day unrelated to actual sleep time. These findings question the generality of the association between long sleep times and mortality because the reports may not have reflected actual sleep. In addition, other lifestyle, health, or socioeconomic factors could be responsible. Finally, it also suggests that interventions to decrease sleep length as a means to increase life span in younger long sleepers might be misdirected. Further research is required to provide clarification regarding the possibility of an independent association between lifetime long sleep patterns and mortality.
In summary, there is a clear association of short sleep times with numerous health problems and increased risk of death. Long sleep durations are also associated with an increased risk of death but underlying causes are less apparent. However, all of these conclusions are limited by the reliance of studies on one or few brief questions related to sleep and later statistical association. More extensive sleep data from objective measures or even a study that follows well-defined short and long sleepers for several years would provide much better insight into sleep duration and health-related outcomes.
Individual Differences in Sleep Duration
The majority of research has assumed that normal adults sleep for 7-8 hours each night. Although this statement is generally true, it is also the case that each individual has a unique amount of sleep that is required to be awake and alert during the day and this amount changes across the lifespan. For example, newborns typically sleep 16 or more hours per day. Sleep time then declines during childhood and adolescence and is more stable through adulthood. In a recent study of age-related changes (31), groups of young (mean 22 years) and older (mean 68 years) healthy individuals were put in an isolated environment where they were scheduled to sleep for 12 hours at night followed by 4 hours in an afternoon nap for several days. Both groups slept more during the first 24 hours (about 12 hours for the young adults and 9.5 hours for the older individuals). The amount of sleep then decreased to about 9 total hours in the younger group including about an hour in the nap and 7 hours in the older participants, also including about an hour in the nap. These data demonstrate a large difference in maximal ability to sleep as a function of age, and an inability to extend sleep beyond these values despite spending 16 hours per day in bed. The younger and older groups had reported habitual sleep times of 8 and 6 - 7 hours, respectively, upon entry into the study. Therefore, both groups slept about an hour longer in the isolation environment but this was at the expense of spending almost eight additional hours in bed. In the real world, it is likely that time spent attempting to sleep is determined by the balance between "cost and benefit," where the benefits in alertness and performance associated with increased sleep are balanced by the cost of spending additional time in bed (and, perhaps, additional time awake in bed).
Researchers have also identified people who had short (6 hours or less) or long (9 hours or more) sleep requirements throughout much of their lives (32). In general, long sleep patterns are more stable and persistent throughout life while short sleep patterns more often begin in the late teens (33). Both long and short sleepers became sleepier and have poor performance after total sleep deprivation (34), although some of the short sleepers may have been partially sleep deprived (35). Other research has identified rare extremely short sleepers who have documented sleep of less than three hours per night but do not display daytime sleepiness or performance decrements associated with chronic sleep loss (36), (37). The existence of extremely short sleepers has led to the hypothesis that sleep could be an instinct rather than a restorative process (38). There is little recent support for this theory, but sleep durations are widely divergent across animal species and our knowledge of sleep function remains limited.
There are significant mood, performance, health, and mortality consequences associated with sleep restriction, and these consequences increase as sleep restriction becomes chronic. Although several studies have shown increased risk of death is also associated with sleep duration of 9 hours or more, a clear explanation has not been presented for this, and attempts to shorten sleep lengths in otherwise normal long sleepers are not advised. There are significant individual differences in required sleep length so that incremental changes over time, such as those that may be due to aging, are not be pathological. However, changes in sleep requirement or daytime alertness can also be associated with many common medical conditions such as sleep apnea, diabetes, or thyroid dysfunction and should always be discussed with a physician.
Any discussion of sleep need that cannot explain why humans need sleep at all is not completely satisfying. Research suggests that sleep is a favored time for many types of restoration and renewal, but, as many systems benefit, there may not be a single sleep duration requirement even within an individual. Additionally, sleep that is frequently disturbed and therefore of poor quality cannot be evaluated by a simple duration number. Such caveats simply imply that sleep is a rich and still poorly understood phenomenon. However, current studies of the genetic composition of long and short sleepers and individuals more and less sensitive to sleep loss may allow better identification and stratification of groups to follow throughout life to understand outcomes more clearly. The genetic work may also provide associations with genes controlling other functions, and this might help to identify the specific role of sleep.
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Sleep duration as a function of age (data from (29))