Individual tolerance to caffeine varies due to genetic differences in adenosine receptors, requiring personal experimentation.

“All of this has to do with the relationship between adenosine and these adenosine receptors, genetic variation, things that are very hard to find out except experimentally, meaning each of you needs to decide and figure out for yourselves whether or not you can tolerate caffeine and at what times of day you can tolerate caffeine in order to still fall asleep easily and get good sleep, so rather than demonize caffeine, or say that everyone can drink caffeine until late, you need to figure out what's right for you.”