A feeling like jetlag that happens after you use your computer after sunset and the bright backlight tricks your body into thinking it's still daytime.
(For details, google "photoentrainment".)
I was online until midnight last night, and now I feel like I'm still on Tokyo time. Netlag sucks.
Disruption of sleep patterns (circadian rhythm) caused by continued computer usage late at night or through interaction with users in different timezones.
I've been on instant messenger with friends in Europe every night this week, and now I'm so netlagged I can't get to sleep before 5 AM.
He had such a severe case of netlag from playing World of Warcraft three nights in a row that he fell asleep in class today.