You can listen to Somnium any time of day or night, so if you can't figure out a convenient way to play it while you sleep, feel free to enjoy it during the day. On the other hand, if you want to play it while you sleep, you can experiment with some alternatives. You can always camp out in the room where you have your home theatre system. You will get the best sound this way. If you prefer to sleep in your bedroom, you can route a headphone extension cable from your DVD player into your bedroom, and try sleeping with a light set of open-ear headphones.
Headphones work best for people who can sleep comfortably on their backs without turning much.
You can also try using small powered speakers near your pillow, plugged into the headphone extension cable. External computer speakers should work, although they won't reproduce the low frequencies very well.