Two hours before bedtime
The type of lightbulbs you use also matters. "Cold, harsh white light" - like that found in fluorescent bulbs - "contains a significant blue component, which is most likely to interfere with sleep onset," says Dr. Michael Terman, an expert on light and biological rhythms at Columbia University Medical Center. Blue light, more so than other colors in the light spectrum, suppresses the body's release of melatonin, the hormone that makes us sleepy.
Check lightbulb packaging for the words "soft" or "warm" and for a color temperature of 3000 kelvins or less, which is less likely to trigger insomnia, Terman says.