Caffeine is a compound naturally found in cocoa beans, kola nuts, tea leaves and other plants like guarana. Caffeine is a stimulant that affects your whole body. Caffeine is a vasodilator, meaning it makes your blood vessels slightly larger in diameter. However, caffeine constricts the blood vessels in your brain. This effect is why caffeine can be found in certain pain or headache medications. It is easier to stem the tide of pain to your brain than to ease the pain once the full force has hit.

How would you like to get paid for preventing someone from doing their own job? Football quarterback-tackling metaphors aside, caffeine works by keeping adenosine from doing its job.

Adenosine is a neurotransmitter, a chemical which affects the way nerves function. Adenosine is a by-product of cellular metabolism like sweat is a by-product of hard physical activity. Just as sweat is there to cool us down, adenosine is there to slow us down. The longer a person is awake and active, the more adenosine accumulates until eventually, they feel it is time to sleep. This is the body’s way of self-regulating activity.

Caffeine works by blocking adenosine from reaching its receptors. If adenosine were a key, and its receptor was a lock, then caffeine is a key that is similar enough to adenosine to fit into the lock, but not similar enough to open or close the lock. As long as caffeine is sitting in those adenosine receptors, adenosine cannot get in and cannot signal fatigue.

That said, caffeine is not always able to block out adenosine. If you’ve been awake for more than 16 hours, your brain has done enough activity to generate a whole army of adenosine. At that point, no amount of caffeine is going to be effective enough to make you truly feel alert and awake.

In a situation called “Competitive Inhibition”, caffeine inhibits adenosine because these two molecules are similar enough in shape to both fit into the adenosine receptor. HOWEVER, in this type of inhibition, the odds of caffeine getting to that receptor before adenosine drop (like your eyelids) when caffeine gets outnumbered.

When you are pushing yourself to stay up too long (*and “too long” will vary by individual, but chances are you’ll know that limit when you get to it*), PLEASE don’t rely on an energy drink to help you. At a certain point, your only salvation is either a nap lasting 20 minutes or a full 6 hours of sleep. You have to pick one or the other, because often times naps longer than 20 minutes can be counter-productive due to REM cycle science that I can’t really explain that well here.