Caffeine

• Central nervous system stimulant
• Daily intake maximum of 400mg
• Binds to $A_1$, $A_{2A}$, $A_{2B}$, and $A_{3}$ receptors
• Antagonist of adenosine

Caffeine can be used to hyperfocus on a specific task when used strategically. This works up until about 200mg of caffeine, after which you tend to get more anxious. Caffeine can also help with physical performance.

Taken too close to bed time, caffeine will mess with your sleep schedule, since it’s the antagonist of adenosine.

In terms of a productivity tool, the opposite of this is alcohol; where caffeine can be used to promote hyperfocus, alcohol can be used to promote scatterfocus.