system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: If caffeine blocks the chemicals that make you tired, how does coffee sometimes you more tired than before?
gpt
Caffeine blocks the chemicals that make you feel tired from being detecting your brain, however, it doesn’t replace these chemicals (adenosine). 

These chemicals continue to build up and when the caffeine begins to wear off, the chemicals flood the receptors leading to an increased level of tiredness, much more than you felt prior to consuming the caffeine.