What happens to your brain when you go without sleep

Sleep deprivation can affect how we look, feel and function. We look at what happens if you stay awake for 24, 36, 48 and 72 hours

In recent years, sleep – or our lack of it – has become a national preoccupation – if not a competitive sport. We swap sleep stats with family and friends, comparing how many hours we did (or didn’t) get, how early we got up and what time we went to bed. Such is our commitment to the cause that the UK sleep aid market is now worth over £100 million, with a projected annual growth rate of 6.34 per cent. We’re stocking up on sleep-inducing tea, CBD, magnesium and melatonin, and using smart sleep-tracking devices to monitor and improve our sleep quality. 
According to the Oura UK sleep report 2024, Britons are getting an average of seven hours and four minutes of sleep per night, which is just within the NHS guidelines of seven to nine hours per night. While that’s not terrible, there’s certainly room for improvement – and the law of averages means that many of us are getting significantly less than the recommended amount of shut-eye. 
While even short-term sleep deprivation can affect how we look, feel and function, longer periods without sleep can have more dramatic consequences for both our short- and long-term health.  
It’s not uncommon to miss a night’s sleep. “But after 24 hours without sleep, all of us will start to demonstrate microsleeps. They could last for anything from a couple of seconds to 10 or even 20 seconds, or brief periods of sleep that are automatic and hard to avoid,” says Dr Rebecca Robbins, an assistant professor at the Division of Sleep Medicine at Harvard Medical School.
Although there are unlikely to be any major health problems caused by short-term sleep deprivation, there can still be serious consequences due to deficits in our attention and reaction times. 
For example, one study found that 24 hours without sleep affects the cognitive processes that are crucial for performing many everyday activities, increasing the risk of errors and accidents. Another study found that for each hour of sleep lost, the risk of a motor vehicle crash increased by 13 per cent in the overall population, and 22 per cent in those who did not report excessive feelings of sleepiness – and presumably didn’t even realise that they were sleep deprived.
Dr Sophie Bostock, the founder of The Sleep Scientist says: “Even after 16 or 17 hours awake, you’ll start to see reaction times fall by 5 per cent. The military has done a lot of research in this area and found that, after 24 hours awake, reaction times fall by 12 per cent, the risk of error is quadrupled, and the ability to learn tasks is impaired, along with decision-making and the capacity to process information. After 24 hours awake you will see detrimental impacts on cognitive function and mood, and anxiety levels will also increase.”
When you haven’t slept for 36 hours, cognitive performance and physical health both become more seriously affected. 
“By this point, reaction skills are already impaired, and it gets much harder to process new information – so it probably feels like things are going in one ear and out of the other,” says Dr Bostock. “Even visual skills, like recognition, are affected, and you may experience blurred vision.”
Research indicates that going this long without sleep could lead to raised blood pressure and high levels of inflammatory markers in the blood, as the body pumps out the stress hormone cortisol to keep you alert. This hormonal activity can affect your metabolism, increasing your appetite and leaving you craving sugar and starchy foods. You can expect to feel edgy, irritable and extremely fatigued – to the point where you can’t be bothered with anything. “This is when anxiety turns to apathy,” says Dr Bostock. “You start to see a real decline in motivation.”
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After two days without sleep, not only will you struggle to stay awake, but you will experience difficulty with even simple tasks. As well as feeling anxious and irritable, you may become more impulsive, and experience depression, euphoria and extreme mood swings. In addition, the number of involuntary microsleeps is likely to increase, which can be dangerous if you are trying to go about your day as usual. 
Dr Bostock says: “At this point, near vision is impaired, and people become more argumentative, less patient, and more hostile as the parts of the brain that control stress response and emotional regulation become a lot more sensitive, so the smallest thing can disrupt our emotional regulation and push us over the edge.”
More serious health consequences also begin to creep in. Research suggests that the immune system is compromised after two days without sleep, with the number of disease-fighting NK (natural killer) cells decreasing by 37 per cent after 48 hours without sleep, but returning to normal levels when regular sleep was restored.
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Extreme sleep deprivation kicks in after more than three days of wakefulness. Perception is now compromised, and you may start to experience delusions and hallucinations.
Due to the dangers of extended sleep deprivation, there are very few studies which reveal its effects, as these are now considered unethical. However, research, which involved a group of astronauts who stayed awake for 72 hours, identified increased heart rate, extreme mood changes and a reduced ability to process information. Analysis of historical studies found that visual and auditory hallucinations, disorientation, disordered thinking and depersonalisation (the feeling of observing yourself from a distance) were the most common perceptual changes, with some people experiencing psychosis or delirium. These were largely found to resolve after a period of normal sleep. 
“Some people may experience these types of symptoms sooner than others,” warns Dr Bostock. “There is a build-up of sleep debt. So you might have only been awake for 16 hours, but if that’s on the back of seven days of sleep debt, you may experience issues sooner.”
Our response to sleep deprivation is highly variable, says Dr Bostock. “We know from research that there’s a massive amount of individual variation, most of which is genetic,” she says.
However, studies indicate that insomnia becomes more prevalent with age because of natural changes in circadian rhythms, with older adults more likely to report difficulty staying asleep or waking up early, resulting in a shorter sleep time. 
Oura’s sleep report maps a steady decline in the amount of sleep we get with age, peaking at seven hours and 20 minutes in our 20s, dropping to seven hours and 12 minutes in our 30s, and six hours and 45 minutes in our 60s.
In addition, research from Loughborough University indicates that women have a higher sleep debt than men, losing on average 56 minutes per night, compared to 28 minutes for men. Their research also indicates that women need 20 minutes more sleep per night more than men, as they are more likely to multitask which means their brains need longer to recover. 
So while sleep deprivation affects us all, those of us who already have an established sleep debt, such as women and those in mid-life and beyond, may feel the effects more keenly and take longer to recover. 
In the short term, sleep deprivation can affect mood, performance and cognition. But there’s a growing body of evidence to suggest that long-term sleep deprivation can be extremely detrimental to health. “It has been associated with an increased risk of many mental and physical health issues including depression, anxiety, certain types of cancer, heart disease, stroke, dementia and obesity as well as diabetes,” explains sleep expert Dr Lindsay Browning, author of Navigating Sleeplessness. 
Research also suggests a link between poor sleep and dementia. Dr Bostock explains: “there’s a period of detoxification or ‘brain cleansing’ that happens during sleep, and that’s when the body ‘mops up’ some of the nasty toxins like beta-amyloid and tau, which have been implicated in Alzheimer’s disease.”
In addition, sleep deprivation can also impair both short- and long-term memory. Dr Bostock says: “During the day we’re processing loads of information. A little of it lives in the short-term working memory in the hippocampus, but that has relatively little capacity. During overnight sleep, we move all these memories from this short-term storage into the larger cortex, where it’s laid down for the future. So if you compress someone’s sleep, you’re impairing memory and the ability to take in new information.”
The good news is that it is possible to recover from a period of sleep deprivation, although it takes time – and you may need to change your habits. “It’s not a case of paying back your sleep debt in one night,” says Dr Bostock. “If you have one night of really poor sleep, it will probably take two or three nights to recover. You won’t catch up by grabbing an extra 15 minutes here and there, but every little helps.”
“It’s never too late to improve your sleep patterns,” says Dr Bostock. “If you make the effort to improve your sleep you can experience benefits the next day, and a week or two of getting consistently good sleep is going to make a profound difference.”
“Most people build up a sleep debt Monday-Friday, and often try to catch up at the weekend,” says Dr Bostock. “But constantly shifting your body clock backwards and forwards makes things worse.”
Light from the sun, moon, streetlights and electronics can all interfere with the production of the sleep hormone melatonin. Fit black-out blinds or wear a sleep mask for the best night’s sleep. 
“Aim for at least seven-and-a-half hours in bed, so you have enough time to fall asleep,” says Dr Bostock.
If you’re often disturbed by neighbours or environmental noise, consider playing white noise or a relaxing sleep soundtrack to help mask sudden or disruptive noises. 
“Open your curtains as soon as you wake up,” says Dr Bostock. “This helps to regulate your circadian rhythm. Get as much natural light as you can during the day – if you can’t get outside, try to sit next to a window.”
Create rituals to prepare your mind and body for bed. Try a warm bath, a mug of herbal tea, a meditation or a scented pillow spray to signal that it’s time to sleep.
“Most people build up a sleep debt Monday-Friday, and often try to catch up at the weekend,” says Dr Bostock. “But constantly shifting your body clock backwards and forwards makes things worse.”
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