If you are undiagnosed and unmedicated, you can try a very strong cup of coffee, or an energy drink. Ritalin is also a stimulant, and caffeine has similar effects.

Beware though, if you consume caffeine regularly, this won't work.

My recommendation: drink only water on normal days, drink very strong caffeinated drink in case of emergency.

It does give you a headache later in the day, and/or the day after, so keep that in mind.