Blue light from computer screens stimulates are brain much like the sun would, and can alter our circadian rhythms and make it harder to fall, or stay, asleep.
If you use your computer late at night, check out an app called f.lux.
f.lux makes the colour of your computer's display adapt to the time of day, warm at night and like sunlight during the day.
It's even possible that you're staying up too late because of your computer. You could use f.lux because it makes you sleep better, or you could just use it just because it makes your computer look better.
A few people in the dojo swear by f.lux, and it's a free app.
Download here (available for Mac OS, Windows Linux, iPhone, iPad, Android)