The past couple of nights I have been taking steps to block the light escaping from my 24/7 pc. It is surprising what a few bits of black duck tape can do. LEDs on the network cards and other status lights on the rear of the case are the big offenders, as are the power and activity lights on the front.
The worst one was the blue LED on the motherboard. It was super bright and seemed to escape from every gap in the case. It was tricky to mask off but a pre-formed hood of tape crimped around the LED seems to have done the trick.
The hope is that the reduced ammount of light in my sleeping environment will result in better quality sleep.