During my University days I've found out that I feel rested when sleeping for a number of hours divisible by 1.5, or to put it simply, only multiples of 1.5h. So, if I had to take a nap or work super late a 3h "nap" would've left me fresh and ready to work while a 3.5h or 4h nap would turn me into a zombie. I haven't thought of this in a while since it became a habit for me but seeing the 7.5h you mention along with small time changes making big difference it all came back.