- 01Feeling fine is not the same as being fine
In a 14-day lab study, people on 6 hours or less kept getting worse on attention and memory tests while rating themselves only a little sleepier. Getting used to 5 hours mostly means you stop noticing the cost.
- 02Short sleepers are real, and rare
A short-sleep gene was found in one family, and its carriers still slept about 6.25 hours. Needing an alarm, sleeping in on days off or dozing in the day means you need more than 5 hours.
For almost every adult, 5 hours of sleep is not enough. The American Academy of Sleep Medicine (AASM, the main US group of sleep doctors) and the Sleep Research Society advise 7 or more hours a night on a regular basis. 1 One short night is survivable; most nights at 5 hours builds a sleep debt (a growing shortfall) that shows up in your driving, your thinking and, over years, your health.
How much sleep do adults need?
In 2015, a panel of sleep experts from the AASM and the Sleep Research Society reviewed the evidence and agreed on one number for adults aged 18 to 60: 12
Adults should sleep 7 or more hours per night on a regular basis to promote optimal health.Watson NF, et al. AASM and Sleep Research Society joint consensus statement, 2015 1
The same statement says that sleeping less than 7 hours on a regular basis is linked with poor health outcomes, including weight gain, diabetes, high blood pressure, heart disease, depression and a higher risk of death. 1 Short sleep is common: in a 2014 CDC survey of 444,306 US adults, more than a third said they usually slept less than 7 hours. 2 Use the sleep calculator to turn your wake-up time into a bedtime that gives you 7 or more hours.
What happens if you sleep 5 hours a night?
No study can lock people on 5 hours for years, so the evidence comes from three kinds of research: lab studies that restrict sleep for days or weeks, studies that give healthy people a virus, and long studies that follow thousands of people. Each points the same way. The table puts the numbers side by side, with an evidence grade for how strong each finding is.
| What was measured | Who was studied | What short sleep did | Evidence |
|---|---|---|---|
| Thinking and alertness | 48 healthy adults in a sleep lab, 4, 6 or 8 hours in bed for 14 days | 6 hours or less a night caused deficits equal to up to 2 nights with no sleep at all 3 | Strong (randomized lab experiment) |
| Car crashes | US drivers in police-reported crashes | 4 to 5 hours of sleep in the previous 24 hours: 4.3 times the crash rate of 7 hours or more 4 | Moderate (observational) |
| Catching a cold | 164 healthy adults given a cold virus in quarantine | Under 5 hours: 4.5 times the odds of a cold; 5 to 6 hours: 4.24 times, compared with over 7 hours 5 | Moderate (small, sleep measured with a wrist device) |
| Long-term illness | 7,864 UK civil servants followed for about 25 years | 5 hours or less at age 50: 30% higher risk of two or more chronic diseases than 7 hours 8 | Moderate (observational) |
| Death from any cause | 1.38 million people in 16 studies | Short sleep: 12% higher risk of death, with short sleep defined differently across studies 9 | Moderate (meta-analysis of observational studies) |
Observational studies (studies that watch what happens rather than change anything) can't prove that short sleep causes disease; illness, stress and shift work can cut sleep too. The lab and virus studies are the ones that show short sleep doing harm directly, and they point the same way.
Five hours and driving
The clearest risk is the next day on the road. A 2016 AAA Foundation for Traffic Safety analysis of US crash investigations compared drivers by how much they had slept in the 24 hours before the crash. 4 The risk climbs gently down to 6 hours and then steeply below 5.
You may have read that a short night leaves you "legally drunk". The study behind that idea measured hours awake, not hours slept. In Australian transport workers and soldiers, 17 to 19 hours without sleep made performance on some tests equal to or worse than a blood alcohol level of 0.05%, and longer stretches matched 0.1%. 10 The legal driving limit in most US states is 0.08%. So the comparison is about staying up late after an early start, which often goes along with a 5-hour night.
Why you can feel fine on 5 hours and still be impaired
In the University of Pennsylvania study, adults kept to 6 or 4 hours in bed for 14 days got steadily worse on attention and memory tests, while their own sleepiness ratings rose at first and then barely moved. 3 The authors put it plainly:
Sleepiness ratings suggest that subjects were largely unaware of these increasing cognitive deficits.Van Dongen HP, et al. Sleep, 2003 3
So "I'm used to it" mostly means you have stopped noticing. The test scores did not adapt.
Are some people fine on 5 hours?
A few people are natural short sleepers. In 2009, researchers found a change in a gene called DEC2 in a mother and daughter who both slept short: the two carriers averaged 6.25 hours a night, compared with 8.06 hours for relatives without the change. 6 Only two carriers were found in that study. 6 Even this famous short-sleeper family slept more than 6 hours, not 5.
A natural short sleeper usually has slept short since childhood, wakes without an alarm, doesn't sleep in on days off and isn't sleepy in the day. If you need an alarm, sleep longer at weekends or doze off in the afternoon, your body is telling you it wants more than 5 hours.
Short on sleep, or a natural short sleeper?
How to get more sleep when you're stuck on 5 hours
- 1Fix the wake-up time, then move bedtime earlierKeep the same wake-up time every day, including weekends, and move bedtime earlier by 15 to 30 minutes every few nights until you are asleep 7 or more hours. The sleep calculator does the counting.
- 2Protect the end of the nightREM sleep, the dreaming stage, is longest in the last hours of sleep, so a 5-hour night loses REM first. The guide what is core sleep explains the stages and what your tracker calls them.
- 3Move caffeine and alcohol earlierThe why can't I sleep guide sets out the habits, including no caffeine, alcohol or nicotine for at least 6 hours before bed.
- 4If you lie awake, ask about CBT-IWhen you give yourself the time but can't sleep, that is insomnia, not a schedule problem. The American College of Physicians recommends cognitive behavioral therapy for insomnia (CBT-I, a short program of sleep habits and thinking techniques) as the first treatment for all adults with chronic insomnia. 7
Supplements come a long way behind these steps. Here is how the common ones grade for sleep, including one LifeFrom doesn't sell.
| Option | Evidence grade | What the evidence says |
|---|---|---|
| CBT-I | Strong | First treatment for chronic insomnia in adults (American College of Physicians: strong recommendation, moderate-quality evidence). 7 |
| Melatonin | Not supported for insomnia | The AASM suggests clinicians not use melatonin to treat trouble falling or staying asleep (a weak recommendation). 11 It has other uses, such as jet lag, that this grade doesn't cover. |
| Valerian | Not supported | The AASM also suggests against valerian for insomnia. 11 See valerian and herbal sleep aids. |
| Glycine | Limited | Three small trials of about 3 grams before bed each found better sleep on at least one measure. See glycine. |
| Magnesium | Limited | Very little research, of low quality; expect a small effect at most. See magnesium. |
None of these makes 5 hours enough. Sleeping pills and supplements can interact with other medicines, so check with a doctor or pharmacist before starting one.
Tired even when you sleep more? The tests worth checking
If you get 7 or more hours for a few weeks and are still worn out, the problem may not be the hours. Some causes show up on a blood test; the most common one doesn't.
Sleep apnea and low mood are common and no blood test finds them.
| If you also notice | What it may point to | The test that checks it |
|---|---|---|
| Loud snoring, gasping, morning headaches, dozing in the day | Sleep apnea | A sleep study (at home or in a lab), not a blood test |
| Restless, crawly legs at night, pale skin, breathlessness on stairs | Low iron or iron deficiency, anemia | Ferritin and hemoglobin |
| Feeling cold, weight gain, constipation, dry skin | An underactive thyroid (hypothyroidism) | TSH |
| Waking at night to pee, thirst | High blood sugar (type 2 diabetes) | HbA1c |
| Tingling hands or feet, a vegan diet, metformin or acid-reducing medicines | Low vitamin B12 | Vitamin B12 |
The full list of causes, and when blood tests help and when they don't, is in why am I so tired all the time. If you want your sleep and results read together, see LifeFrom for sleep.
paper · AAA Foundation for Traffic Safety, 2016Acute sleep deprivation and risk of motor vehicle crash involvementThe crash study behind the 4.3-times figure, with the full breakdown by hours slept.aaafoundation.org
Citations
- Watson NF, Badr MS, Belenky G, 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. J Clin Sleep Med. 2015;11(6):591-592.
- Liu Y, Wheaton AG, Chapman DP, et al. Prevalence of healthy sleep duration among adults: United States, 2014. MMWR Morb Mortal Wkly Rep. 2016;65(6):137-141.
- Van Dongen HP, Maislin G, Mullington JM, Dinges DF. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep. 2003;26(2):117-126.
- Tefft BC. Acute sleep deprivation and risk of motor vehicle crash involvement. Washington, DC: AAA Foundation for Traffic Safety; December 2016.
- Prather AA, Janicki-Deverts D, Hall MH, Cohen S. Behaviorally assessed sleep and susceptibility to the common cold. Sleep. 2015;38(9):1353-1359.
- He Y, Jones CR, Fujiki N, et al. The transcriptional repressor DEC2 regulates sleep length in mammals. Science. 2009;325(5942):866-870.
- Qaseem A, Kansagara D, Forciea MA, et al; Clinical Guidelines Committee of the American College of Physicians. Management of chronic insomnia disorder in adults: a clinical practice guideline from the American College of Physicians. Ann Intern Med. 2016;165(2):125-133.
- Sabia S, Dugravot A, Léger D, et al. Association of sleep duration at age 50, 60, and 70 years with risk of multimorbidity in the UK: 25-year follow-up of the Whitehall II cohort study. PLoS Med. 2022;19(10):e1004109.
- Cappuccio FP, D'Elia L, Strazzullo P, Miller MA. Sleep duration and all-cause mortality: a systematic review and meta-analysis of prospective studies. Sleep. 2010;33(5):585-592.
- Williamson AM, Feyer AM. Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication. Occup Environ Med. 2000;57(10):649-655.
- Sateia MJ, Buysse DJ, Krystal AD, Neubauer DN, Heald JL. Clinical practice guideline for the pharmacologic treatment of chronic insomnia in adults: an American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med. 2017;13(2):307-349.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
