Did anyone else have this happen during pregnancy?

I only just started on my third tri and I've been having mild bouts of insomnia for the last month, it's like I can go a couple days on just a couple hours of sleep, and then it switches and I can sleep for 10 or so hours and still be exhausted. So far my doctor told me that it happens, but I'm trying to figure out if it is normal. I thought you usually didn't get insomnia until the baby was just about due, or when it was just too big to get comfortable.