I have only used it once. I took the whole cap which is 400mgs I believe. It was too much for me. I should have only taken half. It did make me very alert and not the least bit sleepy. I took it because I hadnt had much sleep the night before. After the 2nd hour, I did feel a slight bit jittery but not much and it went away after about 30 minutes. My only complaint about it was that I had a very mild headache all day. Nothing that hurt but just annoying. Advil didnt seem to take it away. It lasted about 15 hours. No problem going to sleep with it. I did have an awesome workout that night while on it. Definitely good stuff but everyone has there own threshold as far as dosing goes. With me, half a cap. would have been good.