What sleep deprivation does to you, and how much sleep you need

Sleep deprivation slows your reactions, blurs your attention and weakens your memory, often before you feel tired enough to notice. Kept up night after night, short sleep is linked with weight gain, diabetes, heart disease and depression. Most adults need at least seven hours of sleep a night, on a regular basis (AASM and SRS, 2015).

Much of the research on short sleep reaches the public through Matthew Walker, the Berkeley neuroscientist who wrote the 2017 bestseller Why We Sleep. In 2019, Alexey Guzey argued that the book "consistently overstates the problem of lack of sleep" (Guzey, 2019). Walker answered point by point and accepted some of the errors (Walker, 2019), and the claims that went too far are flagged below.

A higher risk is not a forecast

Sleep psychologist Jade Wu points out that the frightening headlines are mostly about sleep deprivation, meaning too little time to sleep, and not about lying awake despite plenty of time in bed. Worrying about those headlines can itself keep poor sleep going (Jade Wu). A raised risk is also not a certainty: in one dementia study a sleep scientist described on the ZOE podcast, short sleepers carried a higher risk, yet most of them never developed the disease (ZOE).

We all have a bad night of sleep. It doesn't mean that tomorrow you're going to wake up with depression or Alzheimer's disease. (Matthew Walker, The Diary of a CEO)

After a bad night, the NHS advice is to keep your regular sleeping hours instead of sleeping in (NHS). If worry about sleep is what keeps you up, start with our guide to sleep anxiety.

What does sleep deprivation do to you the next day?

The strongest evidence is about the hours after a short night, and much of it describes changes you won't feel happening.

Your attention drops out for seconds at a time

Short of sleep, the brain can slip into microsleeps: brief moments of sleep while you're otherwise awake. The US National Heart, Lung, and Blood Institute (NHLBI) says you can't control them and might not be aware of them. If you've ever driven somewhere and not remembered part of the trip, that may have been one. The same page says studies show sleep deficiency harms your driving as much as, or more than, being drunk (NHLBI). Walker puts it in road terms: a microsleep lasts a second or two, and at 70 mph that's enough time to drift into the next lane (The Diary of a CEO).

You learn less

In Walker's lab, healthy volunteers either slept or stayed up all night, then tried to learn a list of new facts inside a brain scanner. "There was a 40% deficit in the ability of the brain to make new memories without sleep," he said on Huberman Lab (Huberman Lab). That was a full night without sleep, so it shows the direction of the effect more than what one late evening costs.

Your mood and self-control slip

After a poor night, self-control drops, so switching off the streaming service the next evening gets harder and tiredness ends up costing you more sleep, a loop described on the ZOE podcast (ZOE). The NHLBI adds trouble making decisions, controlling emotions and coping with change (NHLBI).

Your body shifts gear

When you don't get enough sleep, ghrelin (the hormone that makes you hungry) goes up and leptin (the one that makes you feel full) goes down. Blood sugar runs higher than normal, which may raise the risk of diabetes, and common infections can be harder to fight off (NHLBI). Walker says that cutting someone to five hours for a single night is enough to switch on the fight-or-flight system, with blood pressure, cortisol and heart rate all rising (The Peter Attia Drive #48).

Check yourself for daytime sleepiness

Of all the sleep deprivation symptoms, daytime sleepiness is the easiest to check. The NHLBI says you might be sleep deficient if you often feel you could doze off while:

• sitting and reading or watching TV
• sitting still in a public place, such as a movie theater, meeting or classroom
• riding in a car for an hour without stopping
• sitting and talking to someone
• sitting quietly after lunch
• sitting in traffic for a few minutes

If you nod off like this even after plenty of time in bed, or someone tells you that you snore loudly or gasp for air in your sleep, mention it to a doctor (NHLBI).

How much sleep do I need?

Adults aged 18 to 60 should sleep seven or more hours a night on a regular basis, according to a 2015 consensus panel convened by the American Academy of Sleep Medicine and the Sleep Research Society. More than nine hours may suit young adults, people recovering from sleep debt and people who are ill (AASM and SRS, 2015). The CDC uses the same figure and covers the other ages (CDC):

The consensus statement adds that sleep need varies with genes, behavior, health and environment, so the general number is a starting point.

Count hours asleep, not hours in bed

Nobody sleeps every minute they spend in bed. In his reply to Guzey, Walker notes that sleep researchers usually regard a sleep efficiency above 85% as healthy. At that rate, seven hours asleep takes about 8 hours 15 minutes in bed, and since efficiency tends to fall below 85% by about age 60, older adults may need longer (Walker, 2019). Cutting the end of the night costs more than the minutes suggest, because dream (REM) sleep is packed into the last hours; our guide to deep sleep vs REM sleep explains why.

Let your days tell you

Wu compares the eight-hour rule to "drink eight glasses of water a day": a tidy headline for a more complicated answer (Jade Wu). She suggests three questions instead:

• Do you function reasonably well during the day?
• Do you feel sleepy during the day?
• Can you fall asleep at your bedtime, or is it too early?

Stop counting and start noticing. (Jade Wu, sleep psychologist)

Don't assume you're a natural short sleeper

Neurologist and sleep physician Guy Leschziner has seen one or two families in his career in which everyone sleeps about four hours a night with no ill effects, and researchers have found genes behind the trait. It's rare, he stresses: "I wouldn't suggest that most people who are sleeping four hours a night think I must be genetically blessed" (The Diary of a CEO).

Which sleep deprivation claims hold up, and which went too far?

Guzey is explicit that he isn't claiming sleep is unimportant. He and Walker disagree about how big the long-term risks are, and how sure anyone can be.

The seven-hour line holds, but risk rises at both ends

Guzey's own reading of the mortality studies is that people who sleep about seven hours tend to live longest (Guzey, 2019). Leschziner describes the same pattern across whole populations: more deaths from any cause, and more heart disease, in people sleeping under seven hours or over about eight and a half (The Diary of a CEO). That curve is why Walker's line "the shorter your sleep, the shorter your life" (The Diary of a CEO) is too simple, and in his reply he accepts that both short and long sleep carry risk. He also notes that the big studies often measured sleep once, sometimes more than 20 years before the deaths were counted, and Guzey adds that the sleep data are mostly self-reported. What survives is an association, which is how the consensus statement words it: regularly sleeping under seven hours is "associated with" weight gain, diabetes, heart disease, depression and a higher risk of death.

The claim that short sleep doubles cancer risk went too far

The book said that routinely sleeping less than six or seven hours a night more than doubles your risk of cancer. Guzey cited a 2018 review of 65 studies, covering more than 1.5 million people, that found no link between sleep duration and cancer risk (Guzey, 2019). Walker's reply narrowed the claim: "Short sleep is by no means associated with a higher risk of all forms of cancer." He pointed to studies tying short sleep to specific cancers and said future editions would reflect the review (Walker, 2019).

You can't bank sleep, but you can start now

Sleep is not like the bank. (Matthew Walker, The Diary of a CEO)

Walker means you can't run up a debt during the week and pay it off later. In his account, after a full night without sleep, people get back only about half of the lost hours, even with unlimited recovery sleep (The Diary of a CEO). The brain does try to compensate: when very sleep-deprived people come into Leschziner's sleep lab, he sees a large increase in their deep, slow-wave sleep. And years of short sleep don't seal your fate. Speaking to Peter Attia, Walker said that "perhaps it's never too late to start sleeping better" (The Peter Attia Drive #49).

Start with one wake time

The mortality studies Guzey cites put the lowest risk at around seven hours, and the habit that makes seven hours possible on most nights is a fixed wake time. In the 1h Protocol for Better Sleep, it's the anchor the rest of your personal protocol is built around. So the first decision is a small one: which wake time could you keep on a Sunday as well as on a Monday?

Not sure what's shortening your nights? The free part of the program finds your sleep pattern and suggests one step for this week. Start free.

Frequently asked questions

What happens if you don't sleep for a day?

Attention lapses and microsleeps become more likely, and in Matthew Walker's lab, healthy volunteers were 40% worse at making new memories after a night without sleep. You won't get all the lost hours back: in Walker's account, people recover only about half of a lost night.

What are the signs of sleep deprivation?

The clearest sign is daytime sleepiness: feeling you could doze off while reading, watching TV, sitting in a meeting or riding in a car. The NHLBI also lists trouble focusing, slower reactions, more mistakes and difficulty managing emotions. If you're sleepy despite plenty of time in bed, or someone notices loud snoring or gasping, talk to a doctor.

Can you catch up on lost sleep?

Only partly. After a short night the brain spends more time in deep sleep, but Walker estimates people regain only about half of a lost night. Sleep psychologist Jade Wu says a slightly longer weekend sleep helps a little, but big lie-ins jet-lag your body clock, so she suggests keeping your wake time within about an hour all week.

Is 6 hours of sleep enough?

For most adults, the evidence says no. Adults aged 18 to 60 should sleep seven or more hours a night, according to the American Academy of Sleep Medicine and the Sleep Research Society. In a lab study, volunteers held to six hours in bed a night for about 11 days had as many attention lapses as people kept awake for 24 hours, and were largely unaware of it.

Sources

This article draws on Matthew Walker's podcast interviews and his 2019 reply to readers, Alexey Guzey's 2019 critique, and public guidance from US and UK health bodies.

• Recommended amount of sleep for a healthy adult (consensus statement, American Academy of Sleep Medicine and Sleep Research Society, 2015)
• Sleep deprivation and deficiency: how sleep affects your health (public health guidance, NHLBI, 2022)
• About sleep (public health guidance, CDC, 2024)
• Sleep apnea: symptoms (public health guidance, NHLBI, 2025)
• Insomnia (public health guidance, NHS, 2024)
• The World's No.1 Sleep Expert: The 6 Sleep Hacks You NEED! Matthew Walker (podcast episode, The Diary of a CEO E228)
• Dr. Matt Walker: using sleep to improve learning, creativity and memory (podcast episode, Huberman Lab Guest Series, 2024)
• Matthew Walker on sleep, Part II of III (podcast episode, The Peter Attia Drive #48, 2019)
• Matthew Walker on sleep, Part III of III (podcast episode, The Peter Attia Drive #49, 2019)
• Matthew Walker's "Why We Sleep" is riddled with scientific and factual errors (independent critique, Alexey Guzey, 2019)
• Why We Sleep: responses to questions from readers (author's response, Matthew Walker, 2019)
• Sleeping patterns can predict future diseases: Dr Guy Leschziner (podcast episode, The Diary of a CEO, 2024)
• Fix your sleep with the Royal Marines' sleep consultant: Dr Sophie Bostock (podcast episode, ZOE Science & Nutrition, 2025)
• FAQs about sleep and insomnia (expert resource page, Jade Wu, undated)