Brendan Perry, Bo Hansson, Eldkvarn, The 3rd and the Mortal, Nick Cave and the Bad Seeds, Current 93, Fields of the Nephilim ("Elizium"), This Empty Flow, In the Woods..., Ulver, Tenhi, Leonard Cohen and Ozric Tentacles are some more or less mellow artists whom I like to listen to while sleeping. And Opeth of course, who are both more and less mellow.
But perhaps you should give Diamanda Galás ("Schrei X" or "Litanies of Satan", for instance) or Aphrodisiac ("Nonsense chamber") a try. Not very mellow, but interesting as music for sleeping..
