The effect is so large, so easy to notice, it is enshrined in the word cold. We get far more colds and flu in the winter (”flu season”) than in the summer. In this excellent interview, epidemiologist Thomas Jefferson asks:
Why, for example, do we not get influenza in the summertime?
All of the possible explanations listed in this Wikipedia article assume that it is cold weather that makes flu more common in winter. However, an impressive 1981 study found that flu peaked during the light minimum, not the temperature minimum, contradicting all of these explanations.
My proposed explanation is that flu is less common in the summer because people sleep better during the summer. They sleep better in the summer because they get more morning light. More morning light causes your circadian system to have a greater amplitude, which means you sleep more deeply. Better sleep –> better immune function. When I started to sleep much better, I stopped getting noticeable colds and flu.
When I wrote my paper it was essentially impossible to test my idea. You need to measure a lot of sleep — and sleep scientists, intent on making it hard to do what they do, have made this nearly impossible. Perhaps it will soon be easier. To begin with, to test my idea you’d need to improve sleep somehow. To get more light exposure during winter is easy enough with a light box but measuring quality of sleep is much harder. Maybe FitBit (which will start shipping in a few months) will make this possible. I tried using SleepTracker to measure my sleep but after a few months I gave up. There were four big problems: 1. The interface didn’t work very well. It was often hard to get the data from the device into my computer. 2. The whole thing wasn’t designed to measure sleep, it was designed to wake you at a better time than you would wake up without it. 3. The way it measured sleep was a secret. 4. The output — the measure of sleep — was binary. All you were told was whether movement was above or below some threshold. And I had no idea how that threshold was determined.