Your eyes get strained from a long time of staring at the monitor.
You have to stop and rest your eyes for several hours. Better go to sleep, if you use your computer at night.
If you feel you can't see things on the monitor "clearly" the next day after using computer for a short period of time, it means you need to give yourself a longer break from the computer.
Get away from it for a couple of days. It'll help.
Happens to me sometimes too.
Btw, if it still happens after a couple of days of rest, go see your eye doctor.