If you, like 40% of Americans, sometimes have trouble falling asleep, consider blaming your computer.

Turns out, melatonin (the sleep hormone) is largely regulated by blue light. That makes evolutionary sense, as the sun gives off blue light during the day, while the moon, and fire, both give off much redder light at night. So your body monitors blue light levels, emitting hormones accordingly, to create a circadian rhythm: tired at night and alert during the day.

Problem is, we screw with those signals on both ends: we spend too many of our waking hours inside, getting less blue light than we should; and then we spend much of our post-sunset evening in front of boxes like computer screens, getting too much blue light.

To fix the day side of things, you’d need to spend more time outdoors, with more of your skin exposed to the sun. Which, during the winter, probably entails moving to Hawaii.

But fixing the evening side of the equation is much easier: just download F.lux, a great little piece of freeware for Macs, PCs, and Linux.

In short, after sunset, f.lux changes the color temperature of your display, from its default 6500k (even bluer than the 5000k of daylight) to something between 2700k and 4200k (depending on whether the rest of your room’s lighting is tungsten, halogen, or fluorescent).

Give f.lux a whirl for a week. Though it may take a few days of adjustment – your screen will look awfully pink/orange to you at first – by week’s end, I’m betting you’ll have a tough time using your computer without it.