First of all, a rainy day tends to be cold and dark... kind of like night time, which makes it easier to sleep.
Second, I have a metal roof, and I love the sound of rain drops on my metal roof.
Yesterday, I had the best sleep I've had in a long time. The computer was still unplugged because of the New Year's party (which was quite wonderful btw), and I didn't need it running to block out incidental noise anyway, because the construction workers were off for New Year's Day. And Charybdis came in and slept next to me.