Some people take modafinil to be smarter or concentrate better without changing their sleep schedules. If you were to combine it with polyphasic sleep, the goal would not be to reduce sleep further.

Modafinil has a very long half life, 12-16 hours. Stimulants that interfere with sleep require scheduling. The shorter the half-life, the easier the scheduling. This is an advantage of nicotine over caffeine and caffeine over amphetamine. Modafinil has an even longer half life, but it doesn't require knowing your bedtime so far ahead.