In short: Sleep deprivation slows you down in traffic, weakens your immune system, increases your appetite and goes together with a higher risk of cardiovascular disease. For most adults, 7 to 9 hours a night is the target.
What research shows
Being awake is sometimes as dangerous as being drunk
After 17–19 hours awake, people perform on reaction and attention tasks as at about 0.05% blood alcohol.
Tip: Tired and still driving? Take a 20-minute power nap or stop. An open window or music does not help.
Source: Williamson & Feyer, 2000, Occupational and Environmental Medicine.
Sleep 4–5 hours? Then you drive 4x as dangerously
Drivers who slept 4–5 hours were more than 4x as likely to crash as drivers with 7+ hours. At 5–6 hours it was almost 2x.
Tip: Before a long drive, count back: go to bed on time the night before and plan a break every 2 hours.
Source: Tefft, 2016, AAA Foundation for Traffic Safety.
4 seconds in which you drive 110 metres blind
A 4-second microsleep at 100 km/h means about 110 metres without conscious control (calculation: 27.8 m/s x 4).
Tip: Heavy eyelids, yawning and wandering thoughts at the wheel are the signal to stop immediately.
Source: definition of microsleep, incl. Johns, 2000, Journal of Sleep Research.
You don’t notice how badly you perform
Two weeks of 6 hours’ sleep caused as much loss of concentration as two nights without any sleep, while participants felt it wasn’t too bad.
Tip: Do not trust your feeling. Test yourself: if you fall asleep within 5 minutes on a quiet afternoon, you have a sleep deficit.
Source: Van Dongen et al., 2003, Sleep.
Junior doctors make 36% more errors when tired
Interns on long shifts made about 36% more serious medical errors than on shorter shifts.
Tip: Need to make an important decision (money, contract, car)? Do not do it after a short night.
Source: Landrigan et al., 2004, New England Journal of Medicine.
One short night slows your immune cells
After a night of only 4 hours’ sleep, the activity of natural killer cells (immune cells) fell in 42 healthy men. After a night of recovery sleep it was back to its old level.
Tip: Feel a cold coming on? Make extra sleep your first step, not your last.
Source: Irwin et al., 1996, FASEB Journal.
Under 6 hours of sleep: 4x as many colds
People who objectively slept less than 6 hours were more than 4x as likely to fall ill after exposure to a cold virus as people with 7+ hours.
Tip: In seasons with lots of viruses, make sure you get at least 7 hours in bed, preferably 7.5–8.
Source: Prather et al., 2015, Sleep.
Your flu jab works less well if you sleep badly
Sleep-restricted participants produced less than half as many antibodies after a flu vaccine as well-rested people.
Tip: Sleep well the night before and the nights after your jab.
Source: Spiegel et al., 2002, JAMA.
Sleep loss = 550 extra calories without noticing
Twelve young men who slept 4 hours ate about 22% (559 kcal) more the next day than after 8 hours of sleep.
Tip: Slept badly? Have fruit, nuts and protein ready, because your appetite will be bigger.
Source: Brondel et al., 2010, American Journal of Clinical Nutrition.
Your hunger hormones get out of balance
After two nights of 4 hours’ sleep, leptin (satiety) was about 18% lower and ghrelin (hunger) about 28% higher. Cravings for carbohydrates rose the most.
Tip: Do not go to the supermarket tired and take a shopping list.
Source: Spiegel et al., 2004, Annals of Internal Medicine.
Diet + poor sleep = losing muscle, keeping fat
In a small study (10 adults) on calorie restriction, people with 5.5 hours of sleep lost about 55% less fat and about 60% more muscle mass than with 8.5 hours of sleep.
Tip: Want to lose weight? Put 8 hours in bed in your plan, alongside your diet.
Source: Nedeltcheva et al., 2010, Annals of Internal Medicine.
Tired? Suddenly you want the crisps
After sleep loss, people had stronger brain responses to unhealthy food and chose high-calorie options more often.
Tip: Go shopping rested or with a list.
Source: Greer et al., 2013, Nature Communications.
One week of 5 hours’ sleep lowers testosterone
In young, healthy men, testosterone fell by 10–15% after one week of 5 hours’ sleep a night.
Tip: Men who want to get fitter: sleep counts as much as training.
Source: Leproult & Van Cauter, 2011, JAMA.
Diabetes-like after 6 nights of short sleep
After 6 nights of 4 hours’ sleep, healthy young men processed glucose about 40% more slowly, a pattern resembling prediabetes.
Tip: A run of short nights in a busy week? Plan two good nights afterwards.
Source: Spiegel et al., 1999, The Lancet.
Short sleepers have a higher risk of heart disease
A meta-analysis found in short sleepers about a 48% higher risk of coronary heart disease and about 15% higher risk of stroke.
Tip: Sleep is as much part of heart health as eating and exercise.
Source: Cappuccio et al., 2011, European Heart Journal.
Under 6 hours in midlife, more dementia
People who slept 6 hours or less at 50 and 60 were about 30% more likely to develop dementia later. This is an association, not a proven cause.
Tip: Treat sleep as an investment for later.
Source: Sabia et al., 2021, Nature Communications.
One bad night and your brain holds on to more waste
After one night of sleep deprivation, more beta-amyloid, a protein involved in Alzheimer’s, was found in the brain.
Tip: Do not make a habit of getting by on half a night.
Source: Shokri-Kojori et al., 2018, PNAS.
Your brain washes itself while you sleep
In mice, the space around brain cells increased by about 60% during sleep, so waste products drained away faster.
Tip: See sleep as maintenance time. Keep a quiet, dark room at night.
Source: Xie et al., 2013, Science (animal study).
Sleep loss = stronger emotional reactions
In sleep-deprived people, the amygdala responded about 60% more strongly to negative images.
Tip: Before an argument, a difficult conversation or a job interview: sleep first.
Source: Yoo et al., 2007, Current Biology.
Insomnia doubles the risk of depression
People with insomnia were about 2x as likely to develop depression later.
Tip: Sleeping badly for weeks? Talk to your GP; don’t assume “it will pass by itself”.
Source: Baglioni et al., 2011, Journal of Affective Disorders.
Sleep loss shows on your face
Raters judged people after sleep loss as less healthy, less attractive and more tired than the same people when rested.
Tip: Beauty sleep is real. One extra hour is cheaper than a cream.
Source: Axelsson et al., 2010, BMJ.
Sleep loss makes you lonelier
People with sleep loss felt lonelier and were seen by others as less approachable.
Tip: Feeling low or withdrawn? Check your sleep first.
Source: Ben Simon & Walker, 2018, Nature Communications.
Stress hormone rises after poor sleep
After a night with little sleep, evening cortisol was more than a third higher.
Tip: Want to be less stressed? Start with the night before.
Source: Leproult et al., 1997, Sleep.
Sleep loss and obesity go together
Short sleepers had about 89% higher odds of obesity in children and about 55% in adults.
Tip: Want to lose weight? Start with sleep.
Source: Cappuccio et al., 2008, Sleep.
Sleep loss makes you less honest
People with sleep loss behaved less ethically at work.
Tip: Team leaders: your team’s sleep is also integrity policy.
Source: Barnes et al., 2011, Organizational Behavior and Human Decision Processes.
Poor sleep = more inflammation
A meta-analysis found that sleep problems are linked to higher inflammation markers (CRP and IL-6).
Tip: For your recovery: sleep, nutrition and exercise go together.
Source: Irwin et al., 2016, Biological Psychiatry.
And your mattress?
Getting enough hours but still waking up tired and stiff? Then your sleeping position may play a part. Read back, side or stomach compared.
Frequently asked questions
What are the effects of sleep deprivation?
Among others, slower reactions, a weaker immune system, more appetite, stronger emotional reactions and, in the long term, a higher risk of cardiovascular disease.
How many hours of sleep is too little?
For adults, regularly sleeping less than 6 hours is a risk in research. The advice is 7 to 9 hours a night.
More sleep tips
- Falling asleep faster
- The ideal bedroom
- Power nap and daytime tips
- Food, drink and sleep
- Worrying and insomnia
- Sleep rhythm and regularity
- Sleep and your body
- Sleep in the family
- Sleep, work and performance
- All sleep tips
General information, not medical advice. If you have persistent sleep problems, snore with breathing pauses or are very sleepy during the day, see your GP. SleepLab One is not a medical device. Updated on 22 October 2026.