[user49598]

The importance and effectiveness of caffeine has a lot to do with your job and lifestyle. If you are a mostly sedentary computer worker, the lack of blood circulation from sitting all day is going to make you super tired and slow your brain down. If you're a more active person, either from daily exercise, standing up at work, or a more physically strenuous job you're going to have more maintained energy levels as your body consumes and uses food and pumps blood harder.

Caffeine in either of these situations is going to enable your body to squeeze out more energy but for the person with more active blood flow and metabolism it's probably going to be overkill and they'll be able to get through the day without constant yawning. On the other hand, if you sit for 8 hours you're going to feel almost extreme tiredness towards the end of the day especially if you over consume for lunch.

I guess what I'm saying is that if you want to have a good energetic, drug free day, you should be taking steps to eat well and keep your blood flow up.

Also, when considering the effects of drugs on your body, all anecdotal evidence, like this comment, should really be regarded with extreme scrutiny if not disregarded all together. A good place to begin learning about caffeine would be here: https://en.wikipedia.org/wiki/Caffeine#Health_effects

[Shorel]

Another anecdote, this is mine:

I made a change in my lifestyle and I can work faster and for about three times the hours I was working before that.

My lifestyle change is not related to caffeine, I stopped eating lots of carbs and now I follow a keto diet.

YMMV, etc.