A can of Coke is most appropriate when one is exhausted. There's just enough caffeine to perk you up, but not enough to make you too hyper.

I remember how during camp, when everyone was buying tiny cups of coffee from the vending machine I would get huge cups of Coke. It helped me re-hydrate (we had serious water shortage) and keep me awake (2 hours of sleep. Boo.)

Spending far too much time in sch. Would like to relax at home one day and just sleep. But the exams are coming soon and I don't have time.