Music to sleep by...

I think I mentioned this to Rimo once, but I have a whole 3 1/2 hour playlist for just this purpose. Granted, not all of it is VGM, but the majority of it is.

I second Tappy's suggestion at Donkey Kong Country. "Aquatic Ambience" can be a little loud, but if kept down, it works really well for me. I like the way it just flows. That, combined with the nice, lengthy chords, sounds very soothing.

Another interesting one, albeit short (not even a minute long) track to consider is the "Stage Loading" theme from Mega Man X8. I like this one not only for the ambience, but also for the simple fact that a track of this kind is pretty uncommon for a Mega Man X game, especially when compared to the rest of that album. It has to be one of the most mellow tracks I've ever heard from the series, in a good way.

Finally, I would try "Submerged Temple (Long)" from Metroid Prime 2. It's a slower, even more mellow arrangement of "Brinstar Red Soil Swampy Area" from Super Metroid. On the rip I got, this track is just over 9 minutes long, so it repeats of course. The dynamics stay the same the whole time. A long repeating song that's a slow arrangement of an already awesome song--I think that adds up to a prime candidate.

I don't think songs have to be boring to put you to sleep, but I do feel there is a line. Some songs I've tried tend to keep me awake because I want to hear a certain part later on, for example.

Jam it back in, in the dark.