It is generally not recommended to sleep in earplugs, as you're risking inducing a tinnitus. Basically everyone has some background noise in his ears/head, but it is ordinarily discarded (not brought to consciousness) by central nerve system as it's much quieter than external sounds - but if you cut them out, you're risking that your brain becomes aware of that sound and bam - suddenly you have tinnitus. And it may very well stay with you for the rest of your life.