A really bad thunderstorm for sure. I use a white noise machine so most noises don’t wake me up, but a couple weeks ago we had an early morning thunderstorm right over us and the thunder shook the bed it was so loud.
Other than that, the alarm clock the first morning of the work week. I always get my hours messed up on my days off, and have trouble getting to sleep that last night. Most weeks, I go to work on about three hours sleep that first day. I get up at 3:00 on a work day, but really prefer around 7:00, which is tough when your shift starts at six and it’s an hour commute to get there.