In an experiment by Sheldon Cohen of Carnegie-Mellon University in Pittsburgh, he and his colleagues followed 193 people by interviewing them every night on the phone for two weeks. Each person was asked how they felt that day. These were their options: lively, happy, cheerful, calm, at ease, sad, unhappy, tense, on edge, angry, or hostile.
Everybody has ups and downs, so the researchers averaged each person's responses over the two weeks to get a general measurement of the person's normal mood.
The researchers then put the volunteers in a quarantined facility and gave each of them nasal drops of either a cold or a flu virus, and then tracked their symptoms for a few days.
So what did they find out? "The people who expressed more positive emotions overall," said Cohen, "were much less likely to become sick with a cold or the flu than those who expressed fewer positive emotions...And when they did get sick, they reported milder symptoms."
Your immune system works better when you're in a better mood. And your immune system does far more for you than preventing you from getting a cold or flu. It is worth taking the time and expending the energy to do things that improve your mood. Here is a good place to start: Top Ten Ways to Raise Your Mood.
Adam Khan is the author of Self-Help Stuff That Works and Cultivating Fire: How to Keep Your Motivation White Hot.