How many hours of sleep does the average person require? The American Academy of Sleep Medicine and the Sleep Research Society recently convened an expert panel which reviewed over 5,000 scientific articles and determined that sleeping less than 7 hours in adults (ages 18-60) was associated with worsening health, such as increased obesity and diabetes, higher blood pressure as well as an increased risk of stroke and heart disease. In addition to increasing the risk for illnesses, inadequate sleep is also linked to impaired general functioning, as evidenced by suppressed immune function, deficits in attention and memory, and a higher rate of errors and accidents. Since at least one third of adults report that they sleep less than 7 hours a day (as assessed by the Centers for Disease Control and Prevention in a survey of 444,306 adults), one can legitimately refer to insufficient sleep as a major public health issue. Even though insufficient sleep and other sleep disorders have reached epidemic-like proportions affecting hundreds of millions of adults world-wide, they are not adequately diagnosed and treated when compared to medical risk factors and conditions. For example, in most industrialized countries, primary care physicians perform annual blood pressure and cholesterol level checks, but do not routinely monitor the sleep duration and quality of their patients.
One reason for this may be the complexity of assessing sleep. Checking the blood cholesterol level is quite straightforward and provides a reasonably objective value which is either below or above the recommended cholesterol thresholds. However, when it comes to sleep, matters become more complicated. The above-mentioned expert panel acknowledged that there can be significant differences in the sleep requirements of individuals. Those who suffer from illnesses or have incurred “sleep debt” may require up to 9 hours of sleep, and then there are also significant environmental and genetic factors which can help determine the sleep needs of an individual. The average healthy person may need at least seven hours of sleep but there probably groups of individuals who can function well with merely 6 hours while others may need 9 hours of sleep. Then there is also the issue of the sleep quality. Sleeping for seven hours between 10 pm and 5 am has a higher quality of sleep than sleeping between 6 am and 1 pm because the latter will be associated with many more spontaneous awakenings and interruptions as well as less slow-wave sleep (a form of “deep sleep” characterized by classical slow wave patterns on a brain EEG recording during sleep). Unlike the objective cholesterol blood test, a true assessment of sleep would require an extensive sleep questionnaire asking details about sleep history and perhaps even recording sleep with activity monitors or EEGs.
Another reason for why insufficient sleep is not treated like other risk factors such as cholesterol and blood pressure is that there aren’t any easy fixes for poor sleep and the science of how poor sleep leads to cognitive deficits, diabetes and heart disease is still very much a topic of investigation.
In the case of cholesterol, numerous studies have shown that cholesterol levels can be effectively lowered by taking a daily medication such as a statin and that this intervention clearly lowers the risk of heart attacks and stroke. Furthermore, the science of how cholesterol causes stroke and heart disease has been worked out quite well by identifying the molecular mechanisms of how cholesterol contributes to the build-up of plaque in the arteries which can then lead to heart attacks and stroke. When it comes to sleep, on the other hand, multi-faceted interventions are required to restore healthy sleep levels. Medications to help patients sleep can be used in certain circumstances for a limited time but they are not a long-term solution. Instead, improving sleep requires individualized solutions such as developing a sleep schedule of fixed bed-times, minimizing the use of digital screens in the bedroom, and avoiding caffeine, large meals, nicotine or alcohol just before bedtime. The complexity of assessing and treating insufficient sleep also makes it very difficult to prove the efficacy of interventions. Controlled clinical studies can demonstrate that a cholesterol-lowering medication reduces the risk of heart attacks by treating thousands of patients with the active medication when compared to thousands of patients who receive a placebo, but how do you test the efficacy of individualized sleep interventions in thousands of patients?
Understanding the precise mechanisms by which insufficient sleep impairs our functioning and health has therefore become a major topic of research with significant advances that have been made in the past decades. Correlative studies which link poor sleep to worse health cannot prove that it is the inadequate sleep which caused the problems, but studies in which human subjects undergo well-defined sleep deprivation for a defined number of hours coupled with EEGs, brain imaging studies and cognitive assessments are providing important insights into how poor sleep can affect brain function. The sleep researcher Matthew Walker at the University of California and his colleagues recently reviewed some of the key studies in sleep research and identified some of the major categories of brain function impairment as a consequence of sleep deprivation:
Several studies of human subjects have consistently shown that sleep deprivation leads to a significant decrease in the ability to pay attention to tasks. Some studies have kept subjects awake for 24 hours at a stretch whereas other studies merely restricted sleep to a few hours a night and monitored the performance. Importantly, one study that restricted sleep to less than 3 hours for one week was able to show that the attentiveness and performance of subjects recovered rapidly once the sleep-deprived subjects were allowed to sleep for 8 hours but it still did not return back to the levels of those without sleep deprivation. This means that the after-effects of sleep deprivation can linger for days even when we start sleeping normally.
The impairment of working memory (the temporary memory we use to make decisions and complete tasks) is another key feature of sleep deprivation. Brain imaging studies have been able to identify specific abnormalities in certain areas of the brain that are critical for the “working memory” function such as the dorsolateral prefrontal cortex and thus provide somewhat objective measures of cognitive impairment. Interestingly, placing magnetic coils around the head of sleep-deprived subjects to initiate TMS (transcranial magnetic stimulation) has been reported to help restore some of the loss of visual memory, however, Walker and colleagues note that the benefits of TMS in sleep deprivation are not always consistent and reproducible.
3. Responding to negative stimuli
Sleep deprivation increases responses to negative stimuli such as fear. For example, when subjects who had one night of sleep deprivation were shown images of weapons, snakes or mutilations, their aversion responses were much stronger than those of control subjects. Hyper-responsiveness of the amygdala, the part of the brain which processes emotional reactions, is thought to be one major element in these exaggerated responses of sleep-deprived subjects.
Walker and colleagues note that not all changes seen in the brain imaging studies are necessarily detrimental. In fact, some of these changes may be adaptations that have evolved to help our brains cope with the stress of sleep deprivation. Even though significant progress has been made in sleep deprivation research, understanding differences between individuals in terms of how and why they respond differently to sleep deprivation, distinguishing the mechanisms of beneficial adaptations in brain function from detrimental responses and also developing new studies that study the effects of chronic sleep deprivation – one that occurs over a period of weeks and months and thus mimics real-life sleep deprivation – instead of the short-term acute sleep deprivation studies that are currently performed in the laboratory are major challenges for sleep researchers. Hopefully, advances in sleep research will lead to a better understanding of sleep health and ultimately also translate into sleep becoming an integral part of medical exams in order to address this burgeoning public health problem.
Watson NF, Badr MS, Belenky G, Bliwise DL, Buxton OM, Buysse D, Dinges DF, Gangwisch J, Grandner MA, Kushida C, Malhotra RK, Martin JL, Patel SR, Quan SF, Tasali E. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. J Clin Sleep Med 2015;11(6):591–592.
Liu Y, Wheaton AG, Chapman DP, Cunningham TJ, Lu H, Croft JB. Prevalence of Healthy Sleep Duration among Adults — United States, 2014. MMWR Morb Mortal Wkly Rep 2016;65:137–141
Krause AJ, Simon EB, Mander BA, Greer SM, Saletin JM, Goldstein-Piekarski AN, Walker MP. (2017). The sleep-deprived human brain. Nature Reviews Neuroscience
Note: An earlier version of this article was first published on the 3Quarksdaily blog.