First off, I have narcolepsy, which was diagnosed about 2.5-3 years ago and due to factors beyond my control, I had to change doctors and was unable to be on my full set of meds for some months. Now, however; I am back on my full script regimen (with limited exceptions to be explained later) of Dextroamphetamine: 30mg twice daily (max legal dosage), Modafinil: 200mg twice daily, Fluoxetine: 20mg twice daily, and Temazepam: 10mg-20mg as needed... On top of all that I drink any energy drinks I can get my hands on and smoke 1-2 cigs every three days or so (working on trying to quit, but not working so well), and given all of that, I can still fall asleep during the day usually. Now, the pharmacy that I have to go through to get my meds has a bunch of extra regulations on how they handle Schedule II medications, which means that in the past in the past year, I have not been on my Dex for a total of a month because of the incompetence of their staff and sheer absurdity of their Schedule II medication refill system. So I called my neurologist and asked him what to do and he said to use the Modafinil as needed and he would refill it early, one of the days I got up to about 2g through out the course of the day. Given it was a day that I had to go out and do stuff like drive and interact with people and such, cause the last thing I wanted was to fall asleep while driving or something. The only additional effects that I felt on this high of a dosage was a slight increase in paranoia, muscle tension (especially in my neck), and the normal effects were increased slightly, though I still got a full night of sleep that night.
Now, as for the standard effects that I have noticed while on this medication are that it does promote a state of being awake, it seems to help with my problem solving abilities and a slight increase in the speed at which I can react to situations (or perhaps my brain just subconsciously assesses the situation and the increase in problem solving affects that, I don't know. All I know is that I can have my arms full of groceries, drop a bottle, and catch it before it hits the ground and without dropping any other items.) One other effect that this substance has (it seems to affect depressants in this way as well as stims, though I don't have enough experience on that side to fully verify this theory) is that it seems to synergize with and amplify meds/chems that act on the CNS in general; it definitely does this with CNS stims from my expirence. Which partially explains the ill effects many people have when mixing with caffeine, it would probably feel a lot like you were experiencing some degree of a caffeine OD, and those are just nasty. One last note, the way that I have described how Modafinil feels when it is active is that it has a very "clean and almost clinically sanitary feeling" to it, where for me Adderall (I was on that before I had to change docs) felt kind of "fuzzy and tacky" and Dex just feels "edgy" comparatively. Hopefully that helps a bit, and if anyone has questions, feel free to ask and I will do my best to help answer them.