NoDoz
"I've used NoDoz plenty of times in the past and would highly recommend to those the need a midday/afternoon energy boost for work or study.

I would usually only take one tablet a day when I do use NoDoz and usually within 30min-1hrs I start to feel the results, keeps me awake, focused and energetic while on the job. 

There are some negatives with NoDoz though:
-if you take them too often their effectiveness starts to wear off e.g you will still feel tired and not alert but you won't be able to sleep.

-NoDoz can interrupt your sleep and keep you awake during the night

-would alway recommended having plenty of water while using NoDoz otherwise you may start to feel dehydrated and may lead to headaches"