Sleep Science
Sleep and the immune system: why you get ill after a bad week
Volunteers deliberately exposed to a cold virus are several times more likely to become infected if they have been sleeping badly. The mechanism runs through cytokines, T-cells and antibody response.
The folk belief that being run down makes you catch things has an unusually direct experimental test, because someone actually did the experiment.
The rhinovirus studies
Sheldon Cohen and colleagues recruited healthy volunteers, measured their sleep for two weeks, then administered rhinovirus drops into the nose and quarantined them under observation to see who developed a cold.
Participants sleeping less than seven hours a night were approximately three times more likely to develop a cold than those sleeping eight or more. Sleep efficiency mattered too — those spending less than 92 per cent of time in bed asleep were around five times more likely to become ill.
A later study by Aric Prather's group used actigraphy rather than self-report and found participants sleeping six hours or less were more than four times more likely to develop a cold than those sleeping more than seven.
These are among the cleanest demonstrations in the field: everyone received the same viral dose, under controlled conditions, and the difference in outcome tracked sleep.
Sleep restriction around vaccination reduces the antibody response. Studies of influenza and hepatitis A and B vaccination have found that people sleeping less than about six hours a night around the time of immunisation produce substantially lower antibody titres, and in some studies the deficit persisted for months. A meta-analysis published in 2023 found the effect was clearer in men than women, which may relate to hormonal influences on immune response.
The practical suggestion — sleep properly in the nights either side of a vaccination — is free and rarely given.
What sleep does immunologically
Several mechanisms are reasonably well described.
Cytokine regulation. Pro-inflammatory cytokines including interleukin-6 and tumour necrosis factor alpha show circadian variation and are influenced by sleep. Acute sleep loss produces a transient increase; chronic sleep restriction is associated with sustained low-grade elevation of inflammatory markers including C-reactive protein — which is one plausible route from poor sleep to cardiovascular and metabolic disease.
T-cell function. Sleep enhances the adhesion capacity of T-cells through effects on integrin signalling — the process by which T-cells attach to and attack infected cells. A study by Stoyan Dimitrov and colleagues found this adhesion was reduced after only a few hours of wakefulness, mediated by stress hormones including adrenaline and prostaglandins, which are lower during sleep.
Immunological memory. Slow-wave sleep in particular appears to support the formation of adaptive immune memory. Jan Born's group has argued for a parallel between the consolidation of neural memory and of immunological memory during deep sleep, both involving the low-cortisol, high-growth-hormone environment characteristic of the early night.
Natural killer cell activity falls measurably after even a single night of partial sleep deprivation.
Why illness makes you sleepy
The relationship runs both ways, and the second direction is under-appreciated.
Infection triggers release of cytokines including interleukin-1 and tumour necrosis factor, which act directly on sleep-regulatory regions of the brain to increase slow-wave sleep. This is not incidental — it is an organised response.
Animal studies suggest it is functional. In rabbits infected experimentally, those that mounted a stronger increase in slow-wave sleep had better survival. Sleep during infection appears to be part of the defence rather than a side effect of feeling unwell.
Which supplies a rare piece of evidence-based advice that everyone already follows: when ill, sleep.
The chronic picture
Beyond acute infection, chronic short sleep is associated with elevated inflammatory markers, and inflammation is implicated in cardiovascular disease, type 2 diabetes, depression and some cancers.
Sleep disorders show the same pattern. Obstructive sleep apnoea produces intermittent hypoxia and repeated arousals, and is associated with systemic inflammation. Insomnia is associated with altered inflammatory profiles.
Causality is difficult to establish in observational data, since inflammation also disrupts sleep. But the experimental studies — where healthy volunteers are restricted under controlled conditions and inflammatory markers rise — establish that the arrow runs at least partly from sleep to immunity.
What is worth acting on
Sleep around vaccination. The clearest actionable finding, and the easiest to implement.
Do not treat sleep as the flexible variable during a demanding period. The weeks people most often sacrifice sleep — deadlines, exams, a new baby, travel — are the weeks they are most exposed and least able to afford reduced immunity.
Treat sleep disorders. Untreated apnoea is a chronic inflammatory stimulus, and it is common and diagnosable.
Sleep when ill, without guilt, and recognise that the sleepiness is doing something.
The overstatement to avoid
Sleep is not an immune intervention that prevents disease. The effect sizes in the rhinovirus studies are large, and they concern susceptibility to a common cold under controlled exposure.
Claims that sleep prevents cancer, or that a specific number of hours confers immunity, run well past what has been shown. The defensible statement is narrower and still worth acting on: sleeping badly measurably impairs several arms of immune function, and the effect appears within days.
