Mark posted a link some time ago to a neat utility called F.lux. It adjusts the color tone of your screen to reduce blue light exposure after sunset. It checks where sunset and sunrise are for your location and makes the switch either suddenly or gradually.

I look at my computer screen right up until bedtime for work and there's no way around it, but since I've started using this utility last week I notice that I get sleepy earlier in the night and it's easier to fall asleep after extended staring at the screen. If anything I am becoming sleepy a little too early, at 8 or 9PM. I'm tempted to start a trial of biphasic sleep where I sleep early and get up for a few hours in the middle of the night to read or get work done, before catching some more winks before the morning.