You can't just ask people how much sleep they want. You have to test how they actually ARE, say by looking at white blood cell count, cortisol levels, memory r

You can't just ask people how much sleep they want. You have to test how they actually ARE, say by looking at white blood cell count, cortisol levels, memory retention, problem solving, and compare different baseline levels of sleep on such terms. What I would like to know is what length of sleep a sedentary adult needs, whether that need N can be met by say (N' < N/2) at night and (N'' < N/2) during a day nap, and what (hopefully positive) affect can be had on reducing that need by say, exercise, diet, or sun exposure.