I believe most if not all of the computer users will use their computer (Facebook-ing, Messaging etc.) during the night time, and possibly until the very late night. And during the usage, you might not feel tired (Of course, who want to stop when you going to kill the boss in Mafia War?), but your eyes tell you that they are tired. Why? Most likely it is due to the inappropriate display temperature of your LCD/monitor, which makes your eyes feel tired after an hour or so of computer usage. This is because at daytime, the screen brightness/colour temperature has no problem for your eyes due to the adequate of lights, but when turned to nighttime, sunset happened, the same screen doesn’t work too well for your eyes anymore because you are now rely on the indoor light sources (Halogen, Fluorescent etc.), and the same screen tends to be too bright for you, thus stresses you eyes. Not only that, several studies shows that when an inappropriate display colour temperature exposed to a person’s eyes, he/she tend to have bad sleeping quality.