I did my first startup when I was ~22 years old and still roughly on my teenage-year "sleep till noon, up till 4" schedule, which actually worked somewhat well (I want to be careful with what I say here because I was in a bunch of ways a total fuckup of a husband at the time) --- when the boy woke up, I was awake to take care of him. It did not cost me any productivity (in fact, it probably snapped me out of a bunch of unproductive A.D.D. spells.)