Caffeine is probably is the most widely used psychoactive product in the world.
Yes, you probably have your portion of caffeine every morning as you rush out the door on your way to your daily activities, but caffeine is to be found in many other substances like tea, cocoa, kola nuts, and guarana too.
Caffeine is added to most energy drinks, sodas and plenty of medications too.
How it works is that it blocks the adenosine receptors of your brain, helping you to not feel so tired anymore.
If you consume quite a low to moderate bit of caffeine, say no more than around 40–300 mg, you can actually increase your attention and alertness.
The above doses are good for people who are generally fatigued.