Why Do We Need Sleep?

Most of us only think about sleep when we are not getting enough of it. A few bad nights in a row and everything feels harder. Your patience runs short, your focus drifts, and small decisions start to feel like large ones. That everyday experience is the clearest evidence there is that sleep does something you cannot manage without. Working out exactly what it does, and why, has kept researchers busy for well over a century.

Sleep is a reversible state in which you stop responding to most of what is happening around you, your muscles relax, and your brain activity shifts into patterns that can be measured on an EEG (electroencephalogram). The reversible part matters. It is what separates sleep from anaesthesia or a coma, and it is why a loud enough noise will still pull you out of it.

It is also an active state rather than a shutdown. Your brain is not switched off overnight. Reviews of sleep physiology describe it as a period during which various biological functions are scheduled, rather than a pause in which nothing much happens.

There is no single agreed-upon explanation for why sleep exists, and researchers have been arguing about it for decades. The National Institute of Neurological Disorders and Stroke says flatly that everyone needs sleep, but its biological purpose remains a mystery. What exists instead is a set of overlapping theories, each with decent evidence behind it and none of them is complete on its own.

The restorative theory holds that sleep exists so the body and brain can repair the wear and tear of being awake. The energy conservation theory points to the drop in energy use during sleep. The adaptive or evolutionary theory suggests that sleep keeps animals still and quiet during the hours when being active is most dangerous. A 2020 review in the Journal of Circadian Rhythms works through these competing explanations and argues that sleep may be better understood as one of several physiological rhythms with a shared evolutionary origin, rather than as a single-purpose behaviour.

What is far less contested is what goes wrong without it. That evidence is strong, consistent and much easier to measure, which is where the rest of this article focuses.

While you sleep, your body runs through a set of maintenance jobs. Here is what the research shows about each of them:

Sleep is when your brain sorts through what you took in during the day and decides what to keep. Both deep NREM sleep and REM sleep play a part. Slow-wave sleep is linked to the overnight consolidation of declarative memory, the facts-and-events kind. REM is more associated with emotional and procedural memory.

The old advice to sleep on a problem has some research behind it. A 2013 study at Lancaster University gave people word puzzles they had failed to solve, then let one group sleep before trying again. The sleep group cracked more of the hard problems than the groups that stayed awake. For the easy problems, sleep made no difference at all.

During deep sleep, the body releases the growth hormone, which drives tissue repair, muscle recovery and protein synthesis. The link is tight enough to be measurable. Growth hormone release tracks slow-wave sleep, with the amount secreted rising in line with how much deep sleep you actually get. This is the mechanism behind the restorative theory, and it is why hard physical days tend to be followed by heavier sleep.

Sleep and the immune system act on each other in both directions. A review of sleep and immune function found that sleep has a specific role in forming immunological memory, the process that lets your body recognise an infection it has met before. That role appears tied particularly to slow-wave sleep.

The practical version is more striking. In one experiment, 164 healthy volunteers had their sleep tracked by a wrist monitor for a week, then had rhinovirus dripped into their noses under quarantine. Those averaging under 6 hours a night were roughly four times more likely to come down with a cold than those getting more than 7 hours.

Even while you are dreaming, your brain is working. During sleep, it clears out waste products that build up while you are awake, and it supports the way nerve cells communicate with each other. Much of the detailed work on this clear-out has been done in mice, and how closely it maps onto the human brain is still being studied.

Your circadian rhythm sets the timing for a range of hormones, including melatonin and cortisol. Cortisol follows a daily curve. It begins climbing during the final hours of sleep, peaks around the time you wake and then tails off across the day to its lowest point at night. Short or broken sleep pushes that curve out of shape.

Appetite hormones take a hit too. In a study of 1,024 adults, habitual sleep of 5 hours rather than 8 was associated with leptin around 15% lower and ghrelin around 15% higher. Leptin tells you that you have had enough. Ghrelin tells you that you have not. It was an observational study rather than a trial, so it cannot prove that short sleep caused the shift. The direction still goes some way to explaining the mid-afternoon raid on the biscuit tin after a rough night.

Sleep duration and heart health tend to track closely. A meta-analysis of 17 studies covering more than 500,000 people found a U-shaped relationship with coronary heart disease. Risk was lowest at around 7 to 8 hours a night, rising by roughly 11% for each hour below that and about 7% for each hour above it. Too little and too much both carry a cost.

The mechanism is still being pieced together. A 2019 study in Nature found that mice with fragmented sleep produced more inflammatory immune cells and developed larger fatty deposits in their arteries. That work has not yet been replicated in people, so treat it as a promising lead rather than a settled finding.

Napping has a more mixed record than you might expect. A Swiss study of 3,462 adults followed for 5 years found that people who napped once or twice a week had fewer heart attacks, strokes, and cases of heart failure than those who never napped. No benefit was observed for those who napped more often. The study was observational, so it cannot show that the naps themselves caused the difference.

There is no single number that suits everyone, and the official guidance is looser than most articles let on. The American Academy of Sleep Medicine and the Sleep Research Society reviewed more than 5,300 studies and concluded that adults aged 18 to 60 should regularly sleep 7 hours or more. Note the wording. That is a floor, not a range. The same panel said it is uncertain whether sleeping beyond 9 hours carries any risk at all. More than 9 hours may suit young adults, people recovering from sleep debt and people who are unwell. The familiar “7 to 9” shorthand is a tidy-up of that, not a rule.

Age, genetics and how physically active you are all shift where you sit. Most of us are below the floor regardless. Our 2026 UK Sleep Survey of 2,000 adults found people spend an average of just over 7 hours in bed, but only 6 hours 22 minutes actually asleep. That gap works out at roughly 49 minutes a night of lying there awake. More than four in ten respondents (44%) reported sleeping just 5 to 6 hours.

Here’s what our sleep expert Sammy Margo says:

“Most adults need between 7 to 9 hours of sleep per night to function at their best. The exact number can vary depending on the individual, but the goal is to wake up feeling refreshed and alert. Consistently getting the right amount of sleep is crucial for your overall health, so try to make it a priority each night.”

Sammy Margo, Dreams’ Sleep Expert

For a fuller breakdown, including the recommended hours for babies and children, read our guide to how many hours of sleep we need. Our Sleep Calculator will work out a bedtime based on when you need to be up.

Each night you move through repeated sleep cycles. Most adults run through 4 to 6 of them, averaging roughly 90 minutes each, with about 75% of the night spent in NREM sleep and most of that in stage N2.

• REM sleep: Short for rapid eye movement. This is the phase most associated with processing emotions, consolidating memory and dreaming. Read our full guide to REM sleep and how much you need.
• NREM sleep: Everything that is not REM, divided into three stages known as N1, N2 and N3. N3 is deep sleep, known as the deepest and most restorative stage of the night.

The proportions shift as the night goes on. Deep sleep is front-loaded into the first few cycles. REM stretches out towards morning, which is why you are most likely to wake mid-dream. Clinical reviews of sleep architecture describe this pattern as fairly consistent across healthy adults.

How much REM and NREM you get also depends on the shape of your sleep. Most people follow a monophasic sleep pattern, taking it all in one overnight block. Others, including many shift workers, run a biphasic pattern split across two periods. For more on how these compare, read about the different types of sleep cycles.

Our 2026 Sleep Survey asked people who sleep badly what keeps them up. Racing thoughts or a busy mind came top at 37%, followed by stress at 28% and being too hot at 24%. Struggling to get comfortable was named by 23%.

That last one is worth sitting with, because it is the most fixable thing on the list. If you wake up aching or spend the night shuffling around looking for a position that works, the surface underneath you is a reasonable place to start. Our Sleepmatch technology measures how your body responds to different mattress tensions, narrows down the options, and lets you browse the full range of mattresses and beds from there.

If stress and anxiety are the problem instead, read our tips on how to calm feelings of anxiety for a better night’s sleep. If it is your partner keeping you awake, you might want to consider a sleep divorce. Late-night scrolling is worth a look too, given how social media affects your sleep.

Sleep disorders are diagnosed medical conditions that affect how much, how well, or when you sleep, to the point that daytime functioning suffers. They sit apart from the ordinary bad nights most people get, and they usually need proper support rather than better sleep hygiene. Here are some of the more common ones:

• Insomnia: Persistent difficulty falling asleep or staying asleep, leading to significant sleep loss and distress. Read more on insomnia and how to beat it.
• Sleepwalking: Getting up and moving around while still asleep. Read about the causes of sleepwalking and how to prevent it.
• Sleep apnoea: Breathing stops and starts overnight, most often because the airway becomes too narrow. Read about the symptoms and causes of sleep apnoea.
• Night sweats: Heavy sweating overnight, often enough to wake you. Read more on what causes night sweats and whether they can be stopped.
• Bruxism: Teeth grinding. commonly linked to stress and anxiety. Find out how to stop grinding your teeth at night.
• Night terrors: More intense than nightmares and mostly affecting children, with physical symptoms such as shouting, thrashing and a racing heart. Read about what causes night terrors and how to stop them.
• Narcolepsy: A brain condition affecting the ability to control when you are awake or asleep, causing sudden sleep attacks and heavy daytime tiredness.

Sleep deprivation covers everything from losing a couple of hours on a Tuesday to the months of broken nights that come with a new baby. Everyone goes through patches of it. It is what happens when the patches stop being patches that causes trouble.

The short-term effects show up fast. In our 2026 UK Sleep Survey, people who slept badly reported low energy (54%), fatigue (51%), low mood (39%), irritability (35%) and poor concentration (34%) the following day.

Cognitive performance takes the clearest hit. A 2023 review of sleep deprivation and cognitive performance describes impairments running right across attention, alertness, working memory, memory consolidation, judgement and decision-making. Reviews of insufficient sleep note that people tend to be poor judges of how much it is affecting them, which is part of why chronic short sleep goes unaddressed for so long.

Over months and years, the picture broadens. The National Institute of Neurological Disorders and Stroke links chronic short or poor-quality sleep to higher rates of high blood pressure, cardiovascular disease, diabetes, depression and obesity. These are associations drawn from population studies rather than proof that short sleep causes each condition on its own.