A little off-topic, but does anyone know why Modafinil is so powerful on willpower (i.e. capacity to power through things one doesn't care about)? Anecdotally it's as though it removes all "willpower barriers" - those little objections one uses to avoid doing things (e.g. "but the water is cold").

I initially put it down to it being used when people are sleep-deprived (as I find willpower is easier when sleep-deprived) but it's even more effective when rested.