There's many reasons for this, but it happened to me last night and I was kind of expecting it. It was because the sun was out, I decided we would have a barbeque, and with that came a lot of chicken, consequently I got a raise from the protein overnight and it was high upon waking. Many people change the way or time they eat during the heat which has a massive effect on overnight levels.
But some other points could be such as dehydration, insulin going off etc.