This makes sense because if you're in pain and fall asleep in real life, you don't wake up every time you feel pain (headaches and such) whereas if someone randomly stabbed you with a needle you'd probably wake up.

However, I feel like both DoT and ground-AoE abilities should wake monsters up, but that could be because I'm used to other MMO's doing it this way. I've been playing for a month and a half and I still feel like I should cast DoT's when I plan on sleeping.