Modafinil is a synthetic stimulant whereas Caffeine is a natural stimulant. Both the components helps to boost activeness and alertness. When taken together both the components Modafinil and Caffeine works synergistically to provide prolonged benefits. However, it should be taken in a proper way to avoid side effects like insomnia or narcolepsy.
Modafinil should be taken in the morning before 10 am. This will help to keep up a constant activeness level with caffeine for the rest of the day. Do not exceed the limit of 200mg Caffeine when administering a 200mg dose of Modafinil.
To know more about Modafinil and Caffeine and if it is safe to be taken together read this blog.