Modafinil eliminates the feeling of being tired, but not the body's need for sleep. Being in sleep deficit weakens the immune system, and I've seen long-term modafinil use without sleep recovery end in pneumonia. So, if I take modafinil to work on a project or stay alert for a long drive, and I finish before the modafinil wears off, I'll go to sleep anyway, because even if my brain doesn't care whether it gets sleep or not, my body needs it. With stimulants like caffeine, getting to sleep before the stimulant wears off is difficult and leads to less sleep.

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.