A recent study reported by CNN says they're not alone. People who play MMORPGs tend to sleep less and spend more time playing than players of other computer games. It may seem obvious, since MMOs are by nature somewhat of a time-sink, but there is undeniably something more to it; any activity can potentially be a time-sink, after all -- so what is it about MMOs that makes people actually sink time?
The answer is up for debate, of course, but one important factor is that WoW's community of players gives the accomplishments within the game a context of reality. The game's goals, dangling in front of us like carrots, would be nearly meaningless if we could only appreciate them in a single-player context, but with a whole realm of other players working alongside us to get them too, they can feel very important. If the choice is between a few hours having dreams you won't remember, or getting a little closer to riding an impressive dragon mount, then certainly sleep can seem boring and useless by comparison.