In an attempt to discover if it's just that the show finishes late, I one day managed to catch an episode on a Saturday afternoon. And lo and behold, I fell alseep during that one as well.
I've had discussions with a friend of mine who is a fan of the show, and funnily enough she's the same. We have differing arguments as to why this occurs during this particular show. (And before anyone says anything, the answer is not "because the show is boring" ;-) )
She believes that the music for the show is very soothing, and thus lulls you to sleep.
I think the main actor (A wonderful British guy called John Nettles) has the most wonderful voice and acting style that you find him is relaxing to watch and listen to that you end up relaxing yourself to the land of nod.
So here I am watching an episode at a relatively early-ish hour, and I'm already yawning. Just once i would like to make it through an entire episode. On the other hand, despite not ever finding out who the killer ever is, I always have the most fantastic sleep.