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.