The way I learned it is the natural alertness cycle is sinusoidal, and by taking stimulants strategically timed one can square it off. That way one can increase total hours of close to peak alertness, as well as make sleep deeper and heavier.
I generalized my personal experience. It may be different for different people, but I'm tired longer the more I use stimulants. But I've also had one to two cups of coffee in the morning every day for about four years now. If I drink more at other times I generally feel worse for a longer period of time and don't gain much in the way of concentration.
