Insomnia?

I couldn't sleep last night. I don't think I've ever really had that happen before - normally I feel drowsy by about 11, 11:30 and I go and listen to This Sceptred Isle or the news and drop off. But last night, the midnight news came and went and I was just lying there, feeling kind of heavy and lazy, but not sleepy. I ended up turning the lamp on, letting the cat in and reading for a couple of hours, after which I managed to get some fitful sleep. I hope that I'm not going to be making a habit of this...