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Are you always sick? Time to check on your sleep

Most people would agree that good sleep helps us tackle our daily tasks and be our best selves. A large body of scientific evidence shows that sleep is vital for optimal health of nearly every system of the body, from cognition and mental health to metabolism and cardiovascular function.

The immune system is no exception. It shields us against infections, helps to heal wounds and even protects against chronic and life-threatening diseases, such as cancer. Proper sleep strengthens the immune system and ensures that we have appropriate protection. On the other hand, evidence indicates that lack of sleep impairs our immune system, and this can make us more likely to get sick.

Lack of sleep increases your chances of catching the common cold

Everybody knows the basics of how to avoid getting one – wash your hands and stay away from sick people. But science tells us there is another powerful trick to minimize your chances of getting the common cold – make sure you sleep enough.

Habitually sleeping less than seven hours a night is associated with increased risk of viral infections, such as the common cold and COVID-19. For example, in the first study of this kind, Cohen et al. (2009) investigated whether sleep affects your chances of catching a cold. It measured sleep of 153 healthy volunteers over two weeks, after which participants received nose drops containing a virus that causes the common cold. Then scientists observed who developed common cold symptoms. It was found that sleeping less than seven hours makes you three times more likely to catch a cold than sleeping eight or more.

These findings are supported by another study from 2015. Researchers measured the sleep of 164 healthy volunteers over a week and showed that those who slept less than six hours a night were more likely to report a cold, compared to people who slept more than seven.

Interestingly, sleep efficiency was also a strong predictor for cold susceptibility. This is calculated as the ratio of total time spent asleep in a night compared to the total amount of time spent in bed while awake (i.e. the time it takes you to fall asleep and time spent awake if you wake up during the night). Participants whose sleep efficiency was less than 92% were five-and-a-half times more likely to develop a cold, compared to participants with an efficiency of 98% or more.

A 2021 study showed that you have a higher chance of catching a cold not only if you sleep too little, but also if you sleep too much. Prather et al. used data from the 2010 and 2015 National Health Interview Survey in the US and compared self-reported sleep duration with reports of colds. The results revealed that people who slept five or fewer hours were 44% more likely to report a cold compared to those who slept for seven or eight. Interestingly, those who reported habitually sleeping nine hours or more were 20% more likely to report a cold. However, when considering this it is important to bear in mind that long sleep duration is often a symptom of an underlying condition rather than a risk factor in itself (which is why you may sleep more when you are sick).

Sleep problems increase the risk of COVID-19 infection

We are currently at the last stretch of COVID-19 pandemic. This disease is caused by the coronavirus known as SARS-CoV-2, which infects us through the respiratory tract. SARS-CoV-2 causes symptoms similar to a common cold and thus sleep could also be an important factor in reducing our chances of contracting COVID-19, by maintaining a healthy and functional immune response.

Although there is not yet much data on the relationship between sleep and COVID-19 infection rates in general public, a 2021 study by Kim et al. investigated whether sleep in healthcare workers from six countries had an effect on the risk of COVID-19 infection. This study is very important as healthcare workers are at especially high risk of COVID-19 infection and also usually sleep-deprived. The results showed that a one-hour longer sleep duration decreased the odds of developing COVID-19 by 12%, while those having sleep problems had an 88% greater risk of contracting it.

Lack of sleep reduces your army of protective immune cells and leads to immune molecule imbalance

But what does too little sleep do to the immune system, causing us to become infected more easily? Scientists have come up with quite a few ideas about what happens to your immune system when you do not sleep enough.

The immune system consists of many different kinds of cells, each type with a specific function. For example, B cells produce antibodies that stick to pathogens in our body and help eliminate them faster. Another type of immune cell, called natural killers (NKs), recognizes cells in our body that have been infected with a virus and kills them. This slows down or even stops the spread of viral infection and reduces our chances of getting sick.

Several studies show that poor sleep disrupts normal levels of immune cells, causing an imbalance that can impair immune response and increase susceptibility to infections. For example, Fondel et al. showed that sleeping for fewer than seven hours compared to sleeping between seven and nine decreased the activity of NKs by 30%. And a small but widely cited article from 1994, Irwin et al, tested 23 healthy males and showed that sleeping for four hours for just a single night reduced NK cell activity by up a staggering 72%.

Interestingly, another study reported an increase in NK cell activity after longer sleep deprivation. This suggested that whereas initially activity of these cells deteriorates due to lack of sleep, it could adapt and rally if poor sleep continues.

Another integral part of the immune system are cytokines, small molecules released by immune and other cells in our body to coordinate the immune system. Cytokines basically tell the immune system what to do – initiate immune response (pro-inflammatory cytokines) or stop it (anti-inflammatory cytokines). For example, when you cut your finger, skin cells in that spot release pro-inflammatory cytokines, signaling to your immune system that it needs to get over to the damaged area right away, seal off the wound, and prevent infection.

Concentrations of cytokines are also delicately regulated by time of day. Some are secreted during the day, others at night. Studies show that habitually sleeping less than six hours disrupts this delicate balance and results in higher pro-inflammatory cytokine secretion. Imbalance in cytokine levels impairs normal immune response and makes you more vulnerable to infections.

What can you do?

Based on current scientific evidence, it is clear that proper sleep is vital to ensuring your immune system functions as it should, protecting you from infection.

Try to the best of your ability to get between seven and nine hours of uninterrupted sleep every night. Of course, we are only human, and sometimes life happens and we all have a bad night. Or two. In the event that you don’t get a good night’s sleep, try to avoid crowded places the following day and make sure to wash your hands and don’t touch your face to reduce your risk of infection. Finally, there is some promising data showing that after a night of too little sleep, either a half-hour nap or longer sleep the following night might help to get your immune system back on its feet. Sleep well and stay healthy!

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