The reason we get tired throughout the day is because a neurochemical called adenosine builds up and passes through receptors, which causes nerve cells to become less active and blood vessels in your brain to dilate. In short, your body gets the message that it’s time to rest. But since caffeine resembles adenosine, it binds to these receptors instead—blocking the actual adenosine from passing through and preventing your body from slowing down.
Though caffeine can keep you awake for a while in this way, it doesn’t cause all that adenosine to vanish forever. “Just because our brain is no longer processing the adenosine doesn’t mean it stops producing it,” Michelle Worley, a registered nurse, told PureWow. “When the caffeine inevitably wears off, you’re left with an adenosine buildup which makes you feel even more tired.”
It’s not unlike getting a burst of energy after eating something sugary, only to crash soon after. And speaking of sugar, if your go-to coffee concoction involves lots of it, that could contribute to post-coffee fatigue, too. There’s also the possibility that caffeine’s diuretic quality can cause dehydration, which can make you feel tired (though some studies have suggested that caffeine doesn’t dehydrate us nearly as much as we’ve been led to believe).
Lastly, you may have built up such a high tolerance for caffeine that a cup of joe just doesn’t perk you up like it used to.