Caffeine

Caffeine blocks adenosine, a molecule that builds up through the day and makes you feel drowsy, which is how it produces alertness and delays the feeling of fatigue. It has a half-life of roughly five hours in most adults, so an afternoon dose can still be affecting sleep well into the evening. European food safety guidance treats up to 400mg a day as a reasonable ceiling for healthy adults, roughly four cups of filter coffee, though individual sensitivity varies considerably and is shaped partly by genetics. It is commonly paired with L-theanine, which appears to soften the jittery edge of caffeine while alertness is largely retained.

Stack Almanac totals your caffeine across coffee, tea, pre-workouts, and standalone supplements, all logged in one place instead of estimated separately.