I’ve run some experiments on myself, and this lasts for days at a time with one tall in the morning. If I keep at it for several days, I will crash early one evening, but still only sleep for 4 hours. It’s a nightmare for someone who typically sleeps 8 hours a night — I just feel fog-brained all the time. That said, workouts are super easy.
If I had a tall in the morning and evening on a regular basis, ugh... I can’t even imagine. I did this once for extra energy during a sports tournament (we won), but I basically had to drink myself to sleep each night.
Multiple doctors have told me that caffeine sensitivity varies from person to person, but I am on the unusually sensitive side.
I am 6' 240lbs, she is 4'6" and 90lbs. I can almost microdose caffeine depending on what I am doing... Sometimes it is an energy drink, sometimes cold brew undiluted, sometimes drop coffee. Shoot, at Starbucks I tend to be a "green eye" type of man and laugh when they ask if I want space for creamer.
Unless I pound back a massive amount of caffeine within 2-3 hours of bed I have no issues sleeping; then again, I have never had issues sleeping and the few times I do forget and drink caffeinated before bed, it only delays me a few hours or results in me reading a novel into the night.
If I knew i really needed to sleep i could at any time.