Quote Originally Posted by Xuvial View Post
Motion sickness really covers a massive range of causes and symptoms.

You could be feeling sleepy simply as a by-product of your eyes and brain getting exhausted of all that movement, particle effects, colors, etc. GW2 is seriously effects-heavy if you have your settings turned up, especially if multiple people are on your screen (World encounters/Dungeons/WvW/PvP) and it becomes impossible to tell what the hell is going on past the explosions/rainbows. I'm glad they're rolling out a fix in the next patch it tones down the effects if a single enemy is being engaged by multiple people so you can at least spot the dodge cues.

Alternatively you could be experiencing a mild form of motion sickness, where your eyes/brain aren't exactly enjoying the experience (trying to shut down) but it's not bad enough to cause you headaches or nausea.
Thanks for the info. I think it's the FOV, since I have a pretty big monitor and I'm sitting quite close. Also, I noticed I had the same issue when playing DayZ in first person, but I found out how to increase the FOV and the sleepiness is gone :P