Unless you are used to working 3rd shift, leaving at midnight is a very bad idea. Realistically, you're not going to get much, if any quality rest just before you leave if you try to force yourself to sleep in the afternoon, when you are used to being awake. You'll be driving when your body thinks it should be sleeping. Plus, you're just going to be exhausted by noon the next day.
I can certainly understand getting up early and getting on the road, but you can overdo it and actually become counter productive. You'd be much better off leaving an hour or so before dawn.
Plus, from Sheboygan, your the fastest way west will involve starting your trip on 2 lane roads and you shouldn't have to worry about traffic at all. You'd want to head west on WI-23 and eventually get on I-90either at Portage or Mauston. Or, if you are heading up through Minneapolis, get on I-94 near Eau Clarie by going through Green Bay and Wausau. In either case, you should be out of the state in 5-6 hours at most.