Actually, where the nadir falls depends on what insulin you're using.
With Lantus, nadirs are typically around +6 (give or take). However, there are some cats who have early nadirs and other late nadirs. And if that isn't confusing enough, most cats will have their nadir at a highly unpredictable time just to make your life interesting. My cat typically had an early nadir (around +3 or +4) except when she didn't.