If it wasnt your imagination john then it could well be a form of sleep paralysis as a few folk have mentioned. Theres something called hynogogic hallucinations that is believed to be part of SP though I have had both and think they are seperate things. Theres a link here about it
http://www.patient.co.uk/showdoc/40002314/
The only other thing I can think it could be if you werent imagining it/dreaming or if it isnt HH is that maybe you are sensitive to spirits....stop laffing everyone..... because of the busy lives most of us lead these days people are becoming less aware of strange things happening around us. When we are sleeping it could be that it is much easier for this kind of thing to register with us.
and if i had known so many people were interested in this kind of thing then I would have put my lucid dreams post up in here. I put it in science cos i didnt want a lot of comments about the 'wet' variety lol