Most of us are told to sleep around eight hours a night. But is this number the same for everyone? Why is getting enough rest even important?
The question “how much sleep do we really need?” does not have a simple answer. Numerous factors influence sleep duration, including age, genetics, lifestyle choices, and gender. Though general guidelines may be helpful, sleep is unique to each individual. Expert recommendations are just one piece of a larger and more complex puzzle.
Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately.
Why Sleep Is Necessary
No living organism can survive without sleep. Chronic sleep loss can have a detrimental impact on physical and cognitive health, while total sleep deprivation causes death. (1) Studies conducted on rats found that total sleep deprivation caused mortality in as few as two weeks. (2) Research on healthy human subjects concluded that just 24 hours of sleep deprivation could induce hallucinations and other schizophrenia-like symptoms. (3)
Sleep has three main functions: to process information, save energy, and help cells recover. During the four sleep stages, important data processing occurs. The brain collects a substantial amount of information during the day. Once asleep, the body solidifies and combines these memories, moving the information from short-term to long-term memory. This process is called “consolidation.” (4)
Memory consolidation typically happens in the N3 stage of sleep, which usually begins forty minutes after first falling asleep. Also known as slow-wave sleep, N3 is the deepest and most restorative of all the sleep stages. Studies show that after a good night’s rest, people tend to retain more information and perform better on memory tasks. (5)
Another function of sleep is to conserve energy and replenish energy stores. The waking brain requires significant power to maintain body temperature and other important bodily functions. Because core body temperature drops 1-2°F during sleep, less energy is needed than during wakefulness. The brain reduces energy output specifically during non-REM sleep, when cognitive activity decreases and dreams are infrequent.
Sleep also encourages muscle repair and strengthens the immune system. The sleeping brain triggers the release of hormones that encourage tissue growth and help muscles recover from exercise. (6) White blood cells are released into the bloodstream to combat viruses and bacteria. (7) In one study, people who slept at least eight hours a night were three times less likely to come down with a cold than those who received seven or less. (8)
Additional research connects adequate sleep with various health benefits. These benefits include:
- Improved brain function, such as concentration and memory
- Clear and healthy skin
- Lowered risk for cardiometabolic disease (9)
- Healthy weight maintenance or loss
- Improved immune function
- Maximized athletic performance
Non-REM sleepA type of sleep that consists of three sleep stages — N1, N2, and N3 — where the sleeper’s breathing and heart rate are slow, blood pressure low, and body relatively still.
How Sleep Quantity Is Determined
There is no one-size-fits-all strategy to determine how much rest every person should get. A large number of factors influence sleep quantity, such as:
- Age: How much sleep a person needs is largely dependent on age. (10) As people grow older and experience multiple developmental changes, sleep patterns fluctuate. Over a typical lifespan, the amount of time spent sleeping decreases. Newborns need the most sleep, while the elderly need the least
- Genetics: The amount of time people spend sleeping is linked to two regions of DNA, suggesting that sleep patterns are influenced by genetic factors. (11) Some healthy adults sleep as much as 9.5 hours each night, while others function well with as few as four. The average adult population needs 7.5 hours of sleep. People with a recently-discovered gene mutation called DEC2 can sleep just four to six hours and be fully functional (12)
- Chronotype: The internal biological clock, also called the chronotype, regulates numerous bodily functions including the sleep/wake cycle. This clock can vary slightly from person to person and helps distinguish early birds from night owls. (13) The clock of a morning person runs slightly faster than 24 hours, while the clock of a night owl runs more slowly. An understanding of one’s individual chronotype can help determine the right sleep and wake times, as well as the ideal amount of rest
- Personal motivation: A strong desire to stay awake can override individual sleep needs, even when the brain signals the body to go to bed. This motivation may stem from such activities as studying, working late, or driving long distances. While motivation might override the need to sleep in the short-term, the sleep drive will eventually prevail (14)
General Sleep Recommendations
The National Sleep Foundation researched and reviewed over 300 current scientific publications to help determine how much sleep most people need. (15) Each recommendation varies by age group.
The National Sleep Foundation Sleep Range Recommendations
Newborns (0-3 months): 14-17 hours
Infants (4-11 months): 12-15 hours
Toddlers (1-2 years): 11-14 hours
Preschoolers (3-5): 10-13 hours
School-age children (6-13): 9-11 hours
Teenagers (14-17): 8-10 hours
Younger adults (18-25): 7-9 hours
Adults (26-64): 7-9 hours
Older adults (65+): 7-8 hours
Similar research suggests that healthy adults should receive at least seven hours of sleep each night to promote optimal health. (16) Healthy sleepers spend about 90 percent of the time in bed asleep, meaning eight hours in bed amounts to 7.2 hours of sleep.
Factors That Affect Sleep Duration
Personal health and lifestyle choices can determine sleep quantity. To help calculate ideal sleep and wake times, consider the following tips:
- Listen to the body. Pay attention to productivity and energy levels throughout the day. Does the body feel well rested or tired? Tuning into the body can be a useful tool to determine sleep duration. For many adults, seven hours of sleep is enough to feel energized, while others function better on nine hours of rest
- Check for sleep problems. Symptoms of insomnia, excessive daytime sleepiness, or interrupted sleep may indicate a sleep disorder. Consultation with a sleep professional may help rule out such medical conditions as sleep apnea or narcolepsy
- Examine caffeine consumption. Caffeine is a stimulant that can block sleep-inducing chemicals in the brain and increase adrenaline. Up to 400 milligrams of caffeine — or about four cups of coffee— appears to be safe for most healthy adults. A single cup of coffee or tea may trigger sleep difficulties in those who are sensitive to caffeine
- Understand activity level. Exercise can have a significant effect on sleep requirements. Professional and amateur athletes might need an extra hour or two of sleep to help replenish energy and give muscles time to repair
- Treat other medical conditions and mood disorders. Chronic diseases, pregnancy, depression, and certain medical conditions can have a considerable impact on sleep quantity. A medical professional may help determine proper treatment and prescribe the optimal amount of sleep for each condition
InsomniaInsomnia is a sleep disorder characterized by difficulty falling and/or staying asleep.
Health Risks of Too Little Sleep
According to the American Sleep Association, 35.3 percent of adults report sleeping less than seven hours each night. Getting less than seven hours on a regular basis could lead to excessive tiredness and cause both short and long-term health problems.
In the short term, insufficient sleep can affect judgment, mood, learning, and may increase the risk of serious accidents and injury. (17) People who are sleep deprived might experience daytime drowsiness or episodes of “microsleep.”
Microsleep is a brief period of light sleep — usually lasting only a few seconds — that happens suddenly and without intention. (18) It can be dangerous if it occurs while driving, working in unsafe environments, or watching children who require constant attention.
In the long term, scientific findings associate a lack of sleep with adverse health outcomes. Insufficient sleep prevents the body from strengthening the immune system and producing cytokines, proteins secreted by specific cells to fight infection. (19) Sleep deprivation may delay healing and recovery from illness, and increase the risk of chronic health conditions.
Sleep deprivation could also generate other long-term health problems, such as:
- Weight gain and obesity
- Impaired immune function
Insufficient sleep can also increase the risk of cardiovascular disease, including stroke and atherosclerosis. (20) By contrast, sufficient sleep strengthens the heart vessels and promotes processes that maintain blood pressure and inflammation control.
Can You Sleep Too Much?
Both insufficient and excessive sleep may have a detrimental impact on health. Oversleeping has been connected to medical issues such as diabetes, heart disease, and increased risk of death. (21) Depression and low socioeconomic status also are strongly associated with oversleeping. For example, someone with a lower socioeconomic status may have an undiagnosed illness due to a lack of health care, which may be the cause of excessive sleep.
Oversleeping may also be a sign of a sleep disorder known as hypersomnia. Sufferers may experience extreme sleepiness during the day, excessive sleeping at night, anxiety and depression, and memory problems. (22)
Other possible causes of oversleeping include:
- Alcohol consumption
- Prescription medicine use
- A sudden increase in physical activity
- Neurological disorders, including Alzheimer’s and Parkinson’s disease
Sleep Quality vs. Quantity: The Importance of Both
Quality of sleep is just as important as quantity. Many sleep experts believe 6.5 hours of good sleep is better than eight hours of broken sleep.
For adults, indicators of sleep quality include falling asleep in 30 minutes or less, sleeping soundly through the night, and waking up with relative ease. Signs of poor quality sleep include trouble falling asleep and staying asleep, early awakenings, or trouble waking up.
Sleep quality may be improved by:
- Sticking to a consistent bed and wake time
- Avoiding caffeine in the afternoon
- Keeping the bedroom dark
- Avoiding large meals and alcohol before bed
- Using a white noise or another sound machine (23)
Sleep latencyThe amount of time it takes to fall asleep after the lights have been turned off.
Last Word From Sleepopolis
With so much conflicting information available on sleep duration, it may be difficult to know which numbers to listen to. Scientists have helped define a sleep range to strive for, and this is a helpful start.
When deciding how much sleep we really need, our bodies may tell us exactly what that number should be. Energy levels, physical health, and overall mood can help determine when to sleep and wake, and how much sleep the body requires. Both duration and quality of rest are important keys to daily wellness and, in turn, a healthier life.
- Grandner M, Hale L, et al. Mortality Associated with Short Sleep Duration: The Evidence, The Possible Mechanisms, and The Future. Sleep Medicine Reviews, Jun 2010
- Everson CA, Bergmann BM, et al. Sleep deprivation in the rat: III. Total sleep deprivation. Sleep, Feb 1989
- Petrovsky N, Ettinger U, et al. Sleep Deprivation Disrupts Prepulse Inhibition and Induces Psychosis-Like Symptoms in Healthy Humans. The Journal of Neuroscience, July 2014
- Capellini I, McNamara P, et al. Does Sleep Play a Role in Memory Consolidation? A Comparative Test. PLoS One, Feb 2009
- Potkin K and Bunney Jr W. Sleep Improves Memory: The Effect of Sleep on Long Term Memory in Early Adolescence. PLoS One, Aug 2012
- Dattilo M Antunes HK, et al. Sleep and muscle recovery: endocrinological and molecular basis for a new and promising hypothesis. Medical Hypotheses, Aug 2011
- Besedovsky L, Lange T, et al. Sleep and immune function. Pflügers Archiv: European Journal of Physiology, Jan 2012
- Cohen S, Doyle W, et al. Sleep Habits and Susceptibility to the Common Cold. Archives of internal medicine, Jan 2009
- Chaput J, McNeil J, et al. Seven to Eight Hours of Sleep a Night Is Associated with a Lower Prevalence of the Metabolic Syndrome and Reduced Overall Cardiometabolic Risk in Adults. PLoS One, Sept 2013
- Chaput J, Dutil C, et al. Sleeping hours: what is the ideal number and how does age impact this? Nature and Science of Sleep, Nov 2018
- Parish, J. Genetic and Immunologic Aspects of Sleep and Sleep Disorders. Chest, May 2013
- He Y, Jones C, et al. The Transcriptional Repressor DEC2 Regulates Sleep Length in Mammals. Science, Aug 2009
- Fischer D, Lombardi DA, et al. Chronotypes in the US – Influence of age and sex. PLoS One, Jun 2017
- Lawrence G and Muza R. Assessing the sleeping habits of patients in a sleep disorder centre: a review of sleep diary accuracy. The Journal of Thoracic Disease, Jan 2018
- Hirshkowitz M, Whiton K, et al. National Sleep Foundation’s updated sleep duration recommendations: final report. Sleep Health, Dec 2015
- Watson N, MD, Badr S, et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep, Jun 2015
- Medic G, Wille M, et al. Short- and long-term health consequences of sleep disruption. Nature and Science of Sleep, May 2017
- Priest B, Brichard C, et al. Microsleep during a simplified maintenance of wakefulness test. A validation study of the OSLER test. American Journal of Respiratory and Critical Care Medicine, Jun 2001
- Van der Meide, Schellekens H, et al. Cytokines and the immune response. Biotherapy, 1996
- Mullington J, Haack M, et al. Cardiovascular, Inflammatory and Metabolic Consequences of Sleep Deprivation. Progress in Cardiovascular Diseases, Jul 2012
- Léger D, Beck F, et al. The Risks of Sleeping “Too Much”. Survey of a National Representative Sample of 24671 Adults (INPES Health Barometer). PLoS One, Sep 2014
- Bollu P, Manjamalai S, et al. Hypersomnia. Missouri Medicine, Jan 2018
- Messineo, L, Taranto-Montemurro L, et al. Broadband Sound Administration Improves Sleep Onset Latency in Healthy Subjects in a Model of Transient Insomnia. Frontiers in Neurology, Dec 2017