Twenty-three healthy men were kept awake for the last four hours of a single night. The next day their natural killer cells, the white blood cells that find and destroy cells a virus has taken over, were working at 72% of their usual strength. Then the men slept one ordinary night, and the figure was back where it had started.
Both halves of that finding matter. One short night costs our defences something we can measure. One good night pays it back. The trouble begins when short nights turn into the habit, and there the figures get bigger.
Sleep and immunity work as a two-way system. Sleep keeps the immune system in order, and the immune system, once it's fighting something, sends us to bed.
What happens while we sleep
In 1609 the playwright Thomas Dekker called sleep "that golden chain that ties health and our bodies together". Four centuries on, a few of the links can be named.
Deep sleep changes the body's chemistry. Growth hormone and prolactin rise, and the stress hormones cortisol, adrenaline and noradrenaline fall.
That matters to T cells, the white blood cells that kill infected cells one at a time. To do it, a T cell has to hold on to its target, and it holds on with sticky proteins on its surface. When T cells were taken from people who had slept and from people kept awake through the night, the sleepers' cells were better at switching those proteins on. In the dish, adrenaline and noradrenaline switched them off.
So part of what sleep does for us is clear the stress hormones out of the way.
The opening test was repeated with 42 men, kept awake this time for the first half of the night. Their natural killer cells dipped and came back after one night of recovery sleep, as before. One thing was still low by then: a messenger that tells T cells to multiply. A bad night is repaid quickly, and a little of it lingers. Both tests were run in men only.
Where the line falls for colds
One short night is a dent. A run of them is something else, and it has been tested head-on.
Giving volunteers a cold on purpose is an old method. From 1946 to 1989 the Common Cold Unit near Salisbury advertised its ten-day stays as an unusual holiday.
In the test that matters here, 164 healthy adults wore a wrist sensor that tracked their sleep for a week. Then each was given nasal drops containing a cold virus and kept in quarantine for five days, to see who fell ill. The word is Venetian for forty days, the wait once imposed on plague ships, so these volunteers got off lightly.
Those who had been sleeping under five hours a night were 4.5 times as likely to develop a cold as those sleeping more than seven. Between five and six hours, it was 4.24 times. Between six and seven, the risk was no higher at all.
So the line falls at six hours. With 164 people the exact multiple is rough, and the true figure could be a good deal lower or higher. The direction is what the larger groups below confirm. Age, sex, weight, the season, health habits and the antibodies people started with were all accounted for.
It shows up outside the lab too. Among 651 British Army recruits in thirteen weeks of basic training, those sleeping under six hours a night were four times as likely to be diagnosed with an infection of the nose or throat as those sleeping seven to nine. In a national survey of 22,726 American adults, those on five hours or less reported more colds, and more infections such as flu and pneumonia, over the past month. Those on six hours did not.
Teenagers show the same pattern. Fifty-six of them wore a wrist sensor through about three months of a school term and were asked about their health each week. The longer sleepers had fewer bouts of illness, and in the week before a bout sleep tended to run shorter. That is a small group, and it points the same way as the adults.
What about broken sleep? Here the picture is less clear. In an earlier test, 153 adults kept a sleep diary for two weeks before being given the virus. Those asleep for under 92% of their time in bed were 5.5 times as likely to catch the cold as those at 98% or more. Yet in the wrist-sensor test, broken sleep made no difference, and hours did. For a new parent or a light sleeper, the total we manage is the clearer measure.
One caution about all of these: they measured habits. The 4.5 belongs to a week of short nights. A single bad night before a busy day is the 72%, and that one mends.
"I'm fine on five hours"
A few people are.
In 2009 a family turned up whose short sleepers shared a rare change in one gene, and mice given the same change slept less too. It became known as the short-sleep gene. Then, in 2022, the sleep of 191,929 people was checked against their genes. Those carrying the reported short-sleep variants slept no differently from anyone else.
So the gene is a weak excuse, and the signs are a better guide. A true short sleeper has been this way all their adult life, and it tends to run in the family. They wake after five or six hours feeling rested, with no alarm. Their energy holds through the afternoon. They need no coffee to get going and no lie-in on a day off, and the pattern holds on holiday.
If you need an alarm to wake, coffee to function and two extra hours at the weekend, you are short of sleep and used to it.
Margaret Thatcher is the usual example: she is famously said to have run Britain on four hours a night. Her press secretary, Sir Bernard Ingham, put it more carefully. "She slept four hours a night on weekdays," he said. "I wasn't with her at weekends. I guess she got a bit more then."
Whether a weekend repays the week is doubtful. Young adults held to five-hour nights were given a free weekend. They made up about an hour of sleep in total, and the short nights that followed did their damage all the same. The measures were weight and blood sugar, with nothing on the immune system, so take it as a hint.
Winston Churchill is credited with four hours a night during the war. Less often mentioned is his regular afternoon nap, taken in pyjamas.
The usual advice is seven to nine hours for adults, seven to eight from the age of 65, eight to ten for teenagers, and nine to eleven for school-age children.
More is not always better
It would be neat if every extra hour bought more protection. It does not. Among 56,953 women followed for four years, those sleeping five hours or less had 39% more pneumonia than those sleeping eight. Those sleeping nine hours or more had 38% more.
One reading is that long sleep is a sign more than a cause: a body that is already unwell asks for more. Either way, the aim is enough sleep, and no prize waits for extra.
Night shifts
Sleeping at the wrong hours counts too. From autumn to early summer, 501 health-care workers on rotating or night shifts and 88 on day work logged their symptoms each day, 110,347 diary entries in all. The shift workers had 20% more bouts of flu-like and respiratory illness. The bouts lasted no longer than anyone else's.
If shifts are your life, the Health and Safety Executive's advice is plain. Darken the bedroom with heavy curtains, blackout blinds or eye shades. Ask the household to keep the noise down while you sleep. And if a full sleep is out of reach, rest anyway, "as this is still beneficial".
What about inflammation?
The claim that short sleep sets the whole body alight with inflammation needs trimming. Across 72 sets of findings and more than 50,000 people, those with disturbed sleep had higher levels of two inflammation markers in their blood, and so did the longest sleepers. Yet when sleep was cut short on purpose in the lab, the three main markers did not move.
What a short night does reliably is leave us wired the next evening. In young men, cortisol over the evening hours ran 37% higher after a night cut to four hours, and 45% higher after a night with no sleep at all. It also took at least an hour longer to settle. That may be why the evening after a bad night can feel tired and buzzing at once, and why the early night we planned is harder to get.
If the figures keep you awake
Reading about sleep and illness is a poor lullaby, so this needs saying plainly. One bad night will not destroy you. Your immune system is weakened for a day, and it has backup systems.
The word itself keeps expectations sane. Immune comes from the Latin immunis, exempt from tax or public service. No body is exempt from a cold, rested or not. Sleep improves the odds.
Stop counting. Lying in bed working out how many hours are left if we fall asleep right now wakes us further. Trying to sleep is like trying to force ourselves to relax. The command contradicts itself. Chronic Insomnia: Why Trying Harder Keeps You Awake has what to do instead, and Why You're Still Exhausted Despite Doing Everything Right shows what a sleep tracker's score does to a worried sleeper. For anyone who has got stuck there, lays out a six-week, drug-free programme.
One pattern is worth ruling out. Loud snoring with gasping or choking in the night points to sleep apnoea, and Why Am I Always Tired: When Chronic Exhaustion Has a Cause That Sleep Can't Fix has the signs and the test.
Why being ill makes us sleepy
Now the other direction. Around 350 BC Aristotle wrote that sleep comes from warm vapours rising from the stomach after a meal, and added that people with a fever fall asleep in the same way. He had the mechanism wrong and the observation right.
The modern answer began with dogs. In the early 1900s Ishimori in Japan and Piéron in France each found that fluid from around the brain of a dog kept awake would put a rested dog to sleep. Something was building up in the tired animal, and it was given a name before anyone had seen it: hypnotoxin, a sleep poison.
It took until 1975 to catch one. Fluid from goats and sheep that had been kept awake was given to rested rats and rabbits, and it sent them into deep sleep. Whatever was in it was named factor S, for sleep. Seven years later it was identified, and it was a surprise: a fragment of bacterial cell wall, the debris left when immune cells digest bacteria.
So the first sleep substance ever pinned down was a by-product of the immune system at work. Immune cells answer that debris by releasing messengers, and the messengers act on the brain and deepen sleep. When an infection takes hold, the same messengers rise, and so does the pull towards bed.
Fruit flies carry their own version. A search through more than 12,000 lines of flies turned up a single gene that brings on sleep. Its protein also kills microbes. Flies made to produce more of it slept longer and deeper, and more of them survived an infection. The gene was named nemuri, the Japanese word for sleep.
Does the extra sleep help? Rabbits given a bacterial or fungal infection give a hint. The ones that slept longer afterwards had milder illness and a better outlook, and the ones that died had slept less. Those were rabbits, and the finding shows two things going together and no more. It fits the old advice to sleep it off.
In 1988 Benjamin Hart set out what sick animals have in common: they turn listless, go off their food and stop grooming. He argued that none of it is weakness. It is "an organized, evolved behavioral strategy", and the animal's behaviour is "an all-out effort to overcome the disease".
Fever belongs to the same plan. One view is that the heavy, broken sleep of an infection is built around it: the changes in sleep help the body raise its temperature, and the raised temperature is part of the fight.
So the sleepiness is part of the defence.
A cold steals the sleep it asks for
Here is the unkind part. Twenty-one adults were given a cold virus and had their sleep recorded in a lab for five nights. Once the symptoms arrived, those who fell ill lost 23 minutes of sleep a night, and what they got was more broken.
The dreams change too. When 100 fever dreams were collected in an online survey, they were stranger and darker in mood than ordinary dreams, and more of them were about health and heat.
So illness asks for more sleep and makes it harder to get, and short sleep leaves the door open to the next infection. The way out of that loop is to make sleeping while ill as easy as we can.
Sleeping through a cold
Follow the sleepiness. If you feel tired, sleep. Nap if you need to. Cancel what you can, and ask for help with the rest. Sleep is medicine right now.
For a blocked nose. Sitting in the bathroom with a hot shower running is the usual advice, and it costs nothing. Take it as comfort: six tests of warm, moist air for colds showed no clear gain and no harm. A hot drink has more behind it. In 30 people with colds, a hot fruit drink left the measured airflow through the nose unchanged, yet they felt clearer, and it eased sore throat, chilliness and tiredness as well. The same drink at room temperature did not.
For a cough. Honey. When 105 coughing children were given honey, a cough medicine or nothing before bed, honey did better than nothing, on how often they coughed and on the combined score for the night. The medicine did no better than nothing.
Pooled across 14 sets of findings, honey eased coughs better than the usual care. Your grandma was right about this one. Honey is still sugar, so keep it to a spoonful of on a rough night. It is never for babies under one.
By the bed. Water within reach, and the room cool. How to Boost Energy Levels Naturally: Little-Known Remedies That Actually Work has more on the air and the temperature of a bedroom.
Skip the hot toddy. Alcohol gets us to sleep sooner and then breaks up the second half of the night, the opposite of what a cold needs.
When to get help. A cold that is no better after ten days, a cough that lasts more than three weeks, a very high temperature, or shortness of breath or chest pain: get it checked. The same goes for anyone with a long-term condition or a weakened immune system. Sudden chest pain, severe difficulty breathing or coughing up blood is an emergency.
The day after a bad night
Short nights come for other reasons too: a deadline, a baby, a noisy street. Advice on prevention is no use once the bad night has happened. This is what helps on the day.
Expect less of yourself. After 17 to 19 hours awake, transport and army workers did as badly on some tests as they did with a blood alcohol level of 0.05%, the drink-drive limit in Australia, where the test was run. We would never plan a presentation for after a few drinks, yet we expect ourselves to shine on four hours of sleep. Build in extra time, double-check anything important, and put off big decisions.
Eat protein on purpose. Pooled across 11 experiments, people whose sleep was cut short ate about 385 calories more a day, with more fat and less protein. A short night pushes us towards the sweet and the quick. Eggs or meat at breakfast is the choice to make before the craving makes it for us.
Go easy on the coffee. It hides the tiredness and takes its price at night: a strong coffee six hours before bed still cut sleep by more than an hour. Water is the drink for today.
Take the nap. Eleven young men were held to two hours of sleep. The next day the stress hormone noradrenaline climbed through the afternoon, and an immune messenger in their saliva shifted. When the same men were given two half-hour naps, one in the morning and one in the afternoon, both stayed normal. A helps when the nap has to happen in daylight.
Skip the hard workout. A walk is fine. Hard training on short sleep is the mix the Army recruits were in.
Make tonight the priority. Whatever it takes to get a full night tonight, do it: cancel the evening plans and order in.
At the first scratchy throat, cancel. A sore throat, aches or tiredness beyond the ordinary are signs the fight has started. That is the moment to give the body the sleep it is asking for.
What will not help. A fizzy vitamin drink, fancy mushroom supplements or a $12 cold-pressed green juice can no more stand in for a night's sleep than a protein shake can stand in for going to the gym.
It has been put to the test. Adults held to two hours of sleep a night for three nights took 5.0 days to restore the skin over a small blister wound, against 4.2 days for those who slept. Some of the short sleepers were given extra protein, zinc and vitamins C and D throughout. It raised some immune signals at the wound, and the skin healed no faster. The only real fix is sleep.
Protecting sleep in a busy season
When a busy stretch is coming, these protect the hours.
Set the bedtime in the morning. Count back from when you have to be up, pick the hour, and treat it as an appointment. Left to the evening, it slides.
Keep screens out of bed. People who read on a lit tablet before sleep took longer to drop off, made less melatonin and were less alert the next morning than when they read a printed book.
Air the bedroom. Thirty students slept for a week with the window open or a quiet fan in the vent, and for a week without. With the fresh air they slept better and felt less sleepy the next day.
Mind the two drinks. Coffee late in the day and alcohol in the evening both cost sleep, as above.
Where to start
Go back to the 23 men. Four hours awake took more than a quarter off one line of their defences, and one ordinary night gave it back. We are built to absorb a bad night. A habit of five-hour nights is a different matter, and that one shows up in who catches the cold.
Dekker's golden chain runs both ways. Sleep holds up the defences, and the defences call for sleep when they need it.
The choice was never between sleep and getting things done. A week lost to a cold is a poor trade for the hours we saved.
Protect your sleep, and it protects you. Tonight, give yourself that one ordinary night.
Catching every cold that goes round? Why You Keep Getting Sick: The Causes of Frequent Colds — what else decides who catches a cold, from stress to the people we live with.
Sleeping enough and still worn out? Why You're Always Tired: The Rest You're Missing — the kinds of rest that sleep leaves out, and how to get them.
Know someone who is proud of getting by on five hours? Send them this, and wish them a good night.
Disclaimer: This article is general information. For advice about your own health, talk to a qualified health professional. If your mood has been low for weeks or you're having thoughts of harming yourself, reach out for help today.
Affiliate Disclosure: This article contains affiliate links. As an Amazon Associate, All For Life earns from qualifying purchases, at no extra cost to you. We only link to products we would recommend without the commission.
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