Caffeine may give you that much-needed morning boost, but it can also make you crash—hard. It takes about four cups of coffee to block half of the brain’s A1 receptors. With that many receptors clogged, the adenosine mounting in your body has nowhere to go. So when the caffeine wears off, all that extra adenosine rushes through your receptors. It takes a long time for your body to process the huge flow of new metabolites. And guess what? It leaves you feeling even groggier than you felt before.