Not all colors of light have the same effect. Blue wavelengths — which are beneficial during daylight hours — can disturb sleep at night.

Blue light is found in compact fluorescent and LED lights that are replacing less-efficient incandescent bulbs. Blue light also illuminates smartphones, tablets, and laptops.

A study at Brigham and Women’s Hospital in Boston found that subjects who read e-books on a light-emitting tablet screen for four hours before bed each night took longer to fall asleep and spent less time in REM sleep; had reduced secretion of melatonin, a hormone that induces sleepiness; and had a delayed circadian rhythm. They were also sleepier and less alert the following morning.

Here are some tips for dealing with blue light:

• Expose yourself to lots of bright light during the day.

• Use dim red lights for nightlights.

• Avoid looking at bright screens 2 to 3 hours before bed.