Stumbled across this article the other day:
Why Frasier is the best show to sleep to
It puts words to ideas I've never really thought through, but seems quite accurate! I have some other "sleep" shows--King of the Hill and Everyone Loves Raymond before they were taken off Netflix, and I tried Malcom in the Middle but it just got too wild sometimes--but Frasier has been a consistent one since I started collecting the DVDs ten years ago (and now just use Netflix, of course!). It all makes sense when it's spelled out for me: the musicality, the subdued introduction, the fact that their problems tend to be rather silly and easily-remedied.
For the past few years it's been difficult for me to sleep with the TV on, since my wife is not a TV-sleeper, but between a pair of sleep-headphones and a little tablet PC, I've been able to pull it off again! Woohoo!