It isn't a coincidence: When the weather is better, we feel better, according to researchers (and my own internal barometer). Light that enters our eyes immediately increases levels of mood-boosting serotonin in the brain. Exposure to natural light may also help you control your waistline, says one study. Researchers have noted that those who are exposed to light between 8 a.m. and 12 p.m. have a 20 percent lower BMI than those who had similar food and exercise patterns, but less or later exposure to light.

To reap the benefits, get 20 to 30 minutes of light before noon. If you can't get outside, do what I do: I move my laptop to a sunny spot by the window to make sure my eyes are absorbing plenty of joy-giving light.