Whatever it is, you'll be glad it's not blood pooling - that would surely suggest that your heart was taking a kip at the same time as you.

Usually it's not a symptom of anything awful. It could just be that your eye isn't fully shut when you're asleep - this is common enough. Smoke, alcohol, coughing, sneezing, infections, underactive tear secretion etc. etc. can all add to the effect. If it goes away within a few minutes then you've probably got little to worry about. If your vision was impaired it would be worth getting it checked out.

As usual I agree with Pete <siiigh!> but would like to add that I am a bit uncomfortable with the idea of ignoring possible ill-effects from possible nocturnal lagophthalmos: sleeping with open eyes (like a hare, notionally, which is what the name means!) It is fairly common and I have no medical basis for the discomfort, but it doesn't seem good to expose your cornea and its external mucous membrane to dust, bedclothes, desiccation, and the slings and arrows of outrageous fortune. Also, it would be a good experiment to sleep for a few nights with some means of holding that eye comfortably shut. An eye-pad stuffed with cotton wool or taping a bit of tissue paper over the lid or something like that. If it looked less bloody next day, that would be a good indication.  

Sometimes lagophthalmos may indicate a range of nervous conditions, but if none of them is giving you trouble, you need hardly worry about them before there are other symptoms.