Thousands of years ago, Hippocrates suggested that sleep might improve recovery from illness. Yet despite this ancient observation, it is only quite recently that the link between sleep and our immune system has begun to be elucidated.
Much of our knowledge of the importance of sleep for health emanates from studies examining the effects of sleep loss, and they are compelling. We see that lack of sleep is linked to a whole array of issues: mood disturbances, overeating and weight gain, reduced insulin sensitivity, cardiovascular disease, and many other maladies.
Interestingly, it is also linked to increased susceptibility to respiratory infection – a health concern that is now at the forefront of our minds, perhaps more than ever before. Let’s take a quick look at what the research says about this relationship.
Sleep and Risk for the Common Cold
One study put this to the test in a very direct way. They recruited 153 healthy volunteers and had them report their sleep duration and sleep efficiency (percent of time spent in bed when they are actually asleep) for fourteen days. Then, these brave subjects were administered nasal droplets containing a rhinovirus (the species of virus that is the predominant cause of the common cold). They were quarantined and monitored for five days.
Sure enough, less sleep meant that you were more likely to fall ill. Those with less than seven hours of sleep per night were nearly three times as likely to develop a cold, compared to those who got at least eight hours of sleep. The study also found that people with poor sleep efficiency were 5 and a half times more likely to get the cold. So even if you were in bed for more than eight hours, you may still be at substantially greater risk of getting sick if you spent the night tossing and turning. So, whether it’s too little sleep or poor sleep, not getting the sleep your body needs probably puts you at greater risk for this type of infection.
Sleep and the Response to Vaccinations
Furthermore, inadequate sleep may jeopardize your immune response to vaccination. Observational evidence suggests that people who sleep less show less meaningful responses to influenza and hepatitis B vaccines, compared to those without sleep issues.
This phenomenon has also been demonstrated in controlled studies. When healthy young men had their sleep restricted to just four hours for almost a week, they exhibited a much weaker antibody response to a flu vaccine, compared to counterparts who were allowed to maintain a normal sleep schedule. And healthy adults who got to sleep the night after being vaccinated against hepatitis A had a 2-fold greater antibody titer (concentration) one month later, compared to subjects who were kept awake the night after getting the injection.
Mechanisms Connecting Sleep and Immunity
Why exactly does inadequate sleep lead to poorer immune function? That question does not lend itself to a straightforward answer. The immune system is a very complicated beast, with many different proteins and cells working in concert.
Still, the outcomes we see with sleep and infections are fairly consistent, so we can be confident that something is indeed happening here. A number of studies suggest that reduced sleep correlates with reduced natural killer cell activity.
Natural (born) killer (NK) cells are a type of white blood cell (lymphocyte) that play a crucial role in the innate immune system to fight invading pathogens. They react rapidly to the presence of a virus, so we want these tiny soldiers to be at their best all the time. After a night of partial sleep deprivation, the efficacy of NK cells declines to about 75% of their full strength. So, sleep disturbances make it harder for your innate immune system to mount a defense against invaders.
Sleep seems to also make it easier for immune cells of the learned (adaptive) immune system to latch onto their targets. This brings us to T cells, another type of lymphocyte. They identify their targets (cells invaded by a virus) and release integrin proteins. Integrins enable T cells to bind their target and destroy it. When researchers compared T cells drawn from healthy subjects who have stayed up all night, they found significantly lower levels of integrin activation, compared to that of counterparts who slept.
So good sleep appears one essential contributor to the proper functioning of both the innate and adaptive immune systems.
The underlying molecular and cellular mechanisms that link sleep to immune function are complicated and fascinating, and we don’t fully understand them yet. What we do know is that sleep is a state of restoration. If you skimp on sleep, many of the systems in your body that protect you and keep you functioning at your best, including your immune system, will likely suffer. You will be more vulnerable to catching respiratory viruses, and it will likely take longer to recover from the ensuing illness. Of course, we cannot say for certain whether these findings are broadly applicable to the novel virus that causes COVID-19, but it seems reasonably plausible.
Finally, emerging evidence suggests that chronic sleep loss may be a more serious problem for the immune system, at least in animal models. In fact, acute sleep restriction has been shown at times to ramp up the immune system. This seems paradoxical at first but actually makes sense from an evolutionary standpoint. You can imagine that in a state of total sleep loss in a natural environment, like if you were fleeing from a predator or some other threat, that one might benefit from a temporary boost in the immune and inflammatory response.
However, prolonged partial sleep loss – something that is all too common in the modern world, and more like what people like you and me might regularly experience – appears to be consistently linked to an accelerated decline in immune function. As you might imagine, this is even more relevant to healthcare workers and others in essential services right now, who often must forego sleep or work in shifts, while being exposed to viruses.
There is a lot to say about how to get better sleep but for now, let’s focus on one very simple and effective strategy: make it a priority to go to bed relatively early each night, especially now. This can require some planning and preparation but hopefully, the above information provides you with extra motivation to make this happen.