Lack of sleep: Why not getting enough sleep can make you ill

Why sleep is vital

Sleep is far more than just a rest period. Whilst the body rests, numerous vital processes take place within it. The brain processes the day’s experiences and consolidates new memories. At the same time, hormones are regulated, the immune system is activated and damaged cells and tissues are repaired.

The heart and circulatory system also adjust during sleep. Blood pressure and heart rate drop, taking the strain off the heart. Furthermore, metabolism and blood sugar regulation are affected, whilst inflammatory processes in the body are kept under control.

These regenerative processes are spread across different stages of sleep. Deep sleep, light sleep and REM sleep each fulfil their own functions and complement one another. If sleep is regularly cut short or frequently interrupted, there is less time to complete these processes fully.

How much sleep do people need?

Sleep requirements vary from person to person and change over the course of a lifetime. Most healthy adults need around seven to nine hours of sleep per night. However, it is not just the duration of sleep that matters, but also whether the sleep is regular and restorative.

A study published in *Nature* in 2026 shows that the duration of sleep has a direct impact on health. Researchers analysed data from around half a million people and examined the biological age of various organ systems. In this observational study, both consistently short sleep duration of less than six hours and more than eight hours of sleep per night were linked to accelerated biological ageing of, amongst others, the brain, heart, liver, kidneys and immune system. The most beneficial sleep duration was found to be between approximately 6.4 and 7.8 hours. 

The following table shows the recommended sleep duration according to age group:

What happens in the body whilst we sleep?

During the night, several sleep phases alternate. They repeat four to six times, each fulfilling different functions. It is only through their combined action that complete physical and mental regeneration is possible.

Fall-asleep phase

During the falling-asleep phase, the body prepares for sleep. Heart rate, breathing and muscle tension slowly decrease, whilst the brain gradually transitions from a waking to a sleeping state. Sleep is still light at this stage and can be easily disrupted by even minor stimuli.

Light sleep

Light sleep makes up the majority of the night. The body relaxes increasingly, whilst the brain blocks out external stimuli more and more effectively. At the same time, the processing of initial memory content begins, along with the transition into deeper sleep phases.

Deep sleep

Deep sleep is considered the most important phase for physical regeneration. Heart rate and blood pressure drop, muscles relax and energy consumption decreases. At the same time, growth hormones are released, tissues are repaired and the immune system is supported, amongst other things.

REM sleep

During REM sleep, also known as the dream phase, the brain is particularly active. Memories are consolidated, emotional experiences are processed and learned information is further linked. Although the eyes move rapidly and brain activity is high, the muscles remain largely relaxed.

If sleep duration is regularly reduced, deep sleep and REM phases in particular are more likely to remain incomplete, which can impair important regenerative processes.

What happens after a sleepless night?

Even a single night of insufficient sleep can affect the body and brain. This is initially particularly evident in terms of performance. Many people feel tired, find it harder to concentrate and react more slowly. Mood often changes as well. Irritability, restlessness or reduced resilience are typical consequences.

At the same time, the body adapts to this unfamiliar strain. The stress response is heightened, heart rate and blood pressure may rise, and blood sugar regulation is altered. Feelings of hunger also often increase, particularly the craving for sweet or high-energy foods.

The body can usually cope well with a single short night’s sleep. Most of these changes are temporary and revert to normal after a restful night’s sleep with sufficient rest.

What happens in the brain when you’re sleep-deprived?

The brain is one of the organs that reacts particularly sensitively to sleep deprivation. Even after just one sleepless night, attention, concentration and reaction times decline. Complex tasks become more difficult, decisions become more impulsive and mistakes occur more frequently.

A key reason for this is that important processes involved in memory formation take place during sleep. New information is initially stored temporarily in the hippocampus. Particularly during deep sleep, it is transferred to the neocortex and stored there long-term. At the same time, less important connections between nerve cells are weakened. This allows the brain to conserve energy and free up capacity for new information. Without sufficient sleep, these processes cannot run their full course. This makes learning or remembering difficult.

Emotional processing also changes. Brain regions responsible for attention, planning and self-control function less efficiently, whilst emotional stimuli are perceived more strongly. This can lead to conflicts escalating more quickly, decisions being made more impulsively and moods fluctuating more easily.

Current research: sleep and dementia

A large long-term study showed that people who regularly slept for less than six hours a night between the ages of 50 and 70 had a roughly 30 per cent higher risk of developing dementia later in life.

One possible mechanism under discussion is the so-called glymphatic system. This cleansing system of the brain is particularly active during sleep and helps to remove metabolic waste products. These include the proteins amyloid-β and tau, which play a central role in Alzheimer’s disease. 

Today, many researchers believe that sleep and neurodegenerative diseases influence one another. Chronic sleep deprivation could promote the development of pathological changes. At the same time, early stages of Alzheimer’s disease can impair sleep even years before the first memory problems arise.

What happens to the heart and circulation when sleep is lacking?

The heart and circulatory system also use the night to regenerate. During sleep, heart rate and blood pressure fall. The heart has to work less and the blood vessels can relax. This night-time recovery phase relieves the strain on the cardiovascular system and helps to regulate blood pressure in the long term.

If sleep is lacking, this recovery is partially lost and the body’s stress response is activated. Stress hormones such as adrenaline and cortisol are released in greater quantities, which can cause the heart rate and blood pressure to rise. If this condition persists over a longer period, the cardiovascular system is subjected to increased strain on a permanent basis.

Blood vessels also react to a lack of sleep. Inflammatory processes can increase and impair the function of the inner wall of the blood vessels, known as the endothelium. Among other things, the endothelium regulates vessel diameter and blood flow. If this regulation is impaired, the blood vessels are less able to adapt to changing demands.

Chronic sleep deprivation is now associated with an increased risk of cardiovascular disease. A study published in *Nature Reviews Cardiology* also shows that short sleep is linked to an increased risk of high blood pressure, coronary heart disease, cardiac arrhythmias and strokes.  Sleep deprivation is not usually the sole cause, but it can increase the risk in combination with other factors such as smoking, a lack of exercise or being overweight. 

What happens to your metabolism when you’re sleep-deprived?

Metabolism also changes during sleep. During this time, the body regulates, amongst other things, energy balance and certain hormones that influence hunger, satiety and blood sugar levels. A lack of sleep throws this finely tuned interplay out of balance.

A key reason for this is changes in the hormones that control eating behaviour. The concentration of the hunger hormone ghrelin rises, whilst the satiety hormone leptin decreases. As a result, feelings of hunger increase and, at the same time, it becomes harder to feel full. Many people therefore find themselves reaching for sweet or particularly high-energy foods more often after a short night’s sleep.

A British study involving more than 27,000 adults shows that these hormonal changes are also noticeable in everyday life. People with short or poor-quality sleep reported more frequent bouts of cravings, emotional eating, overeating and regular snacking between meals – regardless of whether they were of normal weight or overweight. 

However, lack of sleep does not only affect eating behaviour. After just a few nights of reduced sleep, the body becomes less sensitive to insulin. As a result, glucose is less easily transported from the blood into the body’s cells, and blood sugar levels rise more readily. If this condition persists over a longer period, the risk of insulin resistance and type 2 diabetes may increase.

What happens to the immune system when we don’t get enough sleep?

The immune system, too, does not use sleep simply to rest, but for important repair and regeneration processes. During the night, numerous immune cells are activated and messenger substances are released to coordinate the body’s defence response. At the same time, immunological memory continues to develop. This helps the body to recognise pathogens it has already encountered more quickly upon renewed contact and to combat them effectively.

Particularly during deep sleep, the composition of various hormones and messenger substances changes. Stress hormones such as cortisol fall, whilst certain cytokines rise. These conditions promote communication between immune cells and support the formation of long-term immunological memory. Adequate sleep therefore also plays an important role in the effectiveness of vaccinations.

A lack of sleep throws this finely tuned interplay out of balance. The immune system reacts less effectively to pathogens, whilst pro-inflammatory processes may increase. In the short term, this increases susceptibility to infections. In cases of chronic sleep deprivation, low-grade inflammation may also persist, which has been linked to various long-term health risks.

A recent Norwegian study demonstrates just how significant this link can be in everyday life. Even a sleep deficit of up to two hours per night was linked to a 33 per cent increased risk of catching a cold. With a greater sleep deficit, the risk increased further. Bronchitis, sinusitis and gastrointestinal infections also occurred more frequently.

Sleep therefore influences not only the strength of the immune system but also its regulation. A restful night’s sleep helps the immune system to combat pathogens effectively and keep inflammatory responses in balance. 

Can you make up for lost sleep?

Many people try to make up for a short night’s sleep at the weekend by sleeping in for longer. In fact, the body can compensate for some of the sleep deficit. After a few nights of insufficient sleep, extra sleep improves, amongst other things, tiredness, alertness and reaction times.

However, sleep cannot be ‘saved up’ at will or fully made up for. Recent studies show that whilst restorative sleep improves mental performance in the short term, it can only partially reverse the biological consequences of prolonged sleep deprivation. Changes to metabolism, the body’s internal clock and individual regenerative processes may persist despite sleeping in.

Anyone who consistently gets too little sleep during the week cannot therefore compensate for this simply by sleeping in for longer at the weekend. It is crucial to maintain as regular a sleep pattern as possible, so that the brain, heart, metabolism and immune system have sufficient time to regenerate every night. Short-term effects such as tiredness or concentration problems can often be alleviated by restorative sleep. However, a consistently reduced amount of sleep should, if possible, not become a habit.

Help with sleep deprivation: 6 tips

Most causes of sleep deprivation cannot be resolved overnight. Nevertheless, even small changes to your sleeping habits can improve sleep quality and give the body more time for its night-time recovery processes. The following tips are well-researched and can usually be easily incorporated into your daily routine.

1. Maintain a regular sleep pattern

Sleep is controlled not only by tiredness, but also by the body’s internal clock. This is located in the brain and synchronises numerous bodily functions with the day-night rhythm. These include, amongst other things, the release of the sleep hormone melatonin, body temperature, the release of cortisol and various metabolic processes.

Irregular bedtimes can throw this internal clock out of sync. As a result, the body is expected to be awake at times when it is actually programmed for sleep – or vice versa. This often makes it harder to fall asleep and stay asleep, and sleep quality suffers.

Maintaining as regular a sleep-wake cycle as possible helps to stabilise these biological processes. It is therefore advisable to go to bed and get up at roughly the same time every day. Even at the weekend, sleeping times should ideally not differ by more than one or two hours from those on weekdays.

2. Make the most of daylight and reduce light in the evening

Light is the most important regulator of the body’s internal clock. Specialised nerve cells in the retina detect the brightness of the surroundings and relay this information to the brain. There, the sleep-wake cycle is synchronised with the natural day-night cycle.

Natural daylight in the morning signals to the body that the day is beginning. This inhibits the release of the sleep hormone melatonin, whilst alertness increases. At the same time, the body’s internal clock is set so that melatonin is released again at the right time in the evening, inducing sleepiness.

Bright artificial light in the evening – particularly the glow from smartphone, tablet or computer screens – can delay this process. The body receives the signal that it is still daytime. Melatonin release begins later, making it harder to fall asleep and potentially shifting the sleep-wake cycle. It is therefore helpful to go outside regularly during the day and to gradually reduce light intensity in the evening.

3. Avoid caffeine and alcohol in the evening

Caffeine and alcohol affect sleep in different ways. Both can reduce the quality of sleep, even if the effects are not always immediately noticeable.

Caffeine blocks the effect of the neurotransmitter adenosine in the brain. Adenosine builds up over the course of the day and increases what is known as ‘sleep pressure’ – that is, the natural urge to sleep. If this effect is inhibited by caffeine, it becomes harder to fall asleep and deep sleep may be shortened. As caffeine is only fully metabolised after several hours, depending on the individual, even a single cup of coffee in the late afternoon can affect sleep that night.

Alcohol initially has a calming effect and helps many people fall asleep. As the night progresses, however, it disrupts the natural sequence of sleep stages. Sleep becomes more restless, night-time awakenings occur more frequently and REM sleep in particular can be impaired. Alcohol is therefore not suitable as a sleep aid.

4. Carefully design your sleeping environment

A quiet, dark and fairly cool bedroom creates the right conditions for restful sleep. Light, noise or a room temperature that is too high can trigger brief moments of waking, even if you do not remember them the next morning.

As you drift off to sleep, your body temperature naturally drops slightly. A room that is too warm can hinder this process. Many people find a temperature between 16 and 19 degrees Celsius comfortable. Good ventilation, a comfortable mattress and a suitable pillow can also help to ensure that sleep is interrupted less frequently.

5. Put your smartphone aside before going to bed

Smartphones and other digital devices can affect sleep in two ways. Firstly, the screen light contains a high proportion of blue light, which can delay the release of the sleep hormone melatonin. Secondly, the content keeps the brain active. Reading news, using social media or watching videos makes it difficult for many people to switch off and prolongs the time it takes to fall asleep.

Ideally, smartphones and tablets should not be used for about 30 to 60 minutes before going to bed. If you use your mobile as an alarm clock, it’s best to put it out of reach to avoid scrolling through your phone in the middle of the night or reaching for it straight after waking up.

6. Opt for light meals in the evening

Digestion also follows a day-night rhythm. Heavy or very high-fat meals shortly before bedtime can put a strain on the body, as the stomach and intestines have to continue working intensively. This makes it harder for some people to fall asleep and can impair the quality of their sleep.

However, you don’t have to go without an evening meal. Easily digestible meals, eaten well in advance of bedtime, are recommended. Those who frequently suffer from heartburn will also benefit from avoiding large, late meals and alcohol as far as possible.

When should a lack of sleep be investigated by a doctor?

A single short night’s sleep or temporary lack of sleep is usually no cause for concern. However, if sleep problems persist for several weeks or significantly affect daily life, the cause should be investigated by a doctor.

A medical assessment is particularly advisable if:

• Difficulty falling asleep or staying asleep persists for longer than four weeks.
• You experience severe tiredness or involuntary microsleeps during the day.
• Concentration, performance or mood are permanently affected.
• Loud snoring, pauses in breathing or gasping for air at night are observed.
• Sleep problems persist despite good sleeping habits.
• You have underlying conditions such as high blood pressure, diabetes or depression, and your sleep is simultaneously deteriorating.

There can be various causes behind persistent sleep deprivation. As well as stress or poor sleeping habits, sleep disorders such as insomnia or sleep apnoea, as well as physical or mental health conditions, may also be to blame. Early assessment helps to identify the cause and treat it specifically.

The first point of contact is usually your GP. They can narrow down possible causes and, if necessary, arrange for further investigations. Depending on the suspected cause, you may then be referred, for example, to a neurologist, an ear, nose and throat specialist, or a sleep medicine centre or sleep laboratory.