An article I wrote last year introduced some new research from Yale University that showed body temperature plays an important role in the process of getting sick from a cold infection. This research showed that a lower body temperature results in a higher replication rate for the virus, leading to a greater chance of becoming sick. The underlying reason for this difference was found in the fact that the cells of our immune system are more mobile at warmer temperatures, allowing them to be more active and destroy invading viruses at a faster rate. If the rate of this activity is faster than the rate at which the virus multiplies, this results in our bodies staving off the infection and defeating it. Now enter new research from University of California, San Francisco (UCSF) that shows sleep may have a similar effect.
The new research examined the cold, flu, pneumonia, and sinus infection rates of 22,000 adults in the United States over a period of seven years. The study catalogued average sleep rates as well. The results were surprisingly straightforward. Compared to those who slept a normal 7-8 hours on an average week night, those who slept less than five hours had a 28% increase in the chance of getting sick from colds and a whopping 82% increase in the chance to come down with flu, pneumonia, or an ear infection. Those who had diagnosed sleeping disorders were also more likely to get sick compared to those without such disorders.
I think many of us can relate to these results. I am sure most of us have had experiences where we stayed up too late working or had a late-night flight, only to come down with a bad sickness the next day. Sometimes, I think I can even "feel" the sickness coming on as a result of my fatigue.
But what is the biological mechanism? How might sleep relate to sickness? The absolutely truthful answer is that we don't know for sure yet. Two likely possibilities, however, are worth considering. The first has to do with the fact that sleep often allows us to experience a long period of sustained warm temperature. Although I do know of people who have bad habits of kicking off their blankets while sleeping, I don't think I have ever heard of anyone who likes to fall asleep cold. Everyone I can think of and everyone I have asked in route to writing this article attests to preferring warmer sleep. So its quite possible this warmth allows the cells of our immune system to function better, much like in the Yale research mentioned above.
A second thing we know about sleep is that our bodies perform a number of maintenance activities during this time. Repairs to torn muscle, mineralization of bone, and clearance of toxins from our brains are all activities known to occur more rapidly during sleep. So another logical possibility is that sleep is also the time for our immune system to become more active in cleaning up infectious materials such as viruses. For the kinds of maintenance activities I have mentioned above, there is usually a signal our body releases as a result of sleep, a hormone that initiates a heightened level of these activities. In this context, it is quite conceivable a similar signal might be used to trigger the cells of our immune system, making them more active than during waking hours. Only future research will tell us for sure but it seems a very plausible, if not likely, possibility.
The bottom line is coming down with a cold or the flu is about more than just being exposed to the virus. Once the virus enters our bodies, our immune cells fight it and have a chance to extinguish it before it multiplies too much and makes us sick. It would appear that warmth and sleep are two critical ingredients in this process, components we should take care to respect. Do note, however, that excess sleep did not seem to have an additional protective effect in the UCSF study. People who slept nine or more hours a day did not have lower rates of cold or flu compared to those who slept only 7-8.
Justin Fendos is a Ph.D. from Yale and a professor at Dongseo University in Busan. He is also the associate director of the Tan School of Genetics at Fudan University, Shanghai, and a National Academy of Sciences Teaching Fellow.
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