Caffeine is a compound that stimulates your central nervous system, making you temporarily feel more alert and awake. To generate these sensations, caffeine has effects on your brain that trigger the release of two key hormones, epinephrine and norepinephrine. Understanding how these hormones affect your body help you understand why caffeine makes you feel more awake.
Adenosine
To trigger hormone release in your body, caffeine molecules first attach themselves to adenosine receptors in your brain. Adenosine’s chief function is to slow down nerve activity, helping you feel tired. When caffeine blocks adenosine from attaching to the receptors, your nervous system starts speeding up instead. This reaction is similar to what happens when you are scared or nervous.
Adrenaline
The increase in nervous system activity that resembles a fear response can signal your brain, specifically the pituitary gland, to release a hormone known as epinephrine. You may be more familiar with epinephrine by the name adrenaline. This hormone is responsible for the fight-or-flight response in your body that could help you evade an attacker or remove yourself from a scary situation. When your adrenaline levels increase, your airways start to open to take in more air and your liver releases glycogen for added energy.
Norepinephrine
In addition to your adrenal gland releasing the hormone adrenaline, your sympathetic nerves will signal the release of the hormone norepinephrine. This hormone works together with adrenaline to increase your heart rate. You may notice your heartbeat speeding up when you drink or eat caffeine. The added blood flow through your body also helps you feel more alert and awake.
Withdrawal
Over time, caffeine in your brain will break free from the adenosine receptors, and the adenosine will reattach to the receptors. This can start to slow down brain activity, which will signal your body to cease producing extra amounts of the hormones epinephrine and norepinephrine. As less and less caffeine is in your body, you will notice the effects of caffeine and hormone withdrawals: your heart rate will slow, your digestion will resume and you may begin to feel more tired. These effects are why MayoClinic.com recommends consuming caffeine in moderation, which is about 200 milligrams of caffeine per day -- roughly the equivalent of two 12-ounce cups of coffee. This can prevent excessively high hormonal fluctuations that cause you to experience jitters, anxiety or stomach upset.
