Yesterday (Monday) was day one of my pre-op diet. I'm allowed 2 protein shakes, a smart meal, and a smart snack per day. I had all but the snack as I didn't need it. However it's almost 5am and I still haven't been able to fall asleep. Is it normal to experience insomnia and restlessness during the pre-op diet?

No, not always. Understand that you are about to undergo a major operation and the fear can cause additional stress and restlessness.

Also you were probably given a requirement to offload caffeine. When I gave up my caffeine habit (6 diet cokes a day), I suffered through a week of caffeine withdrawal syndrome which consisted of severe body aches and headaches. This might also be a potential cause.