I used to do that. I always try and concentrate on wonderful things when I fall asleep. Or I imagine me sleeping with a big gun and shooting all the bad things that come my way.
mmm.. sometimes I also have this one. F.E lately I woke up screaming because in my dream I was fighting and pessed off. I open my eyes and i was terrified. It was so real that i was still feeling this anger.
hahah I rarely worked tho!