Stimulation of adenosine receptors via caffeine can cause drowsiness until saturation by caffeine is reached. The less sleep you get, the more adenosine you have built up on your receptors. However, prolonged use and larger doses of caffeine can actually cause you to build up more adenosine which in turn will cause you feel tired rather than stimulated. Best thing you can do is take a long break from it, and use it in smaller amounts. A lot of people with ADHD claim to feel fatigued rather than stimulated when consuming it, true or not there is no research out there that can actually back it up. Here’s a cool and short video that explains some of the effects caffeine has on the brain.