Caffeine doesn’t just affect your brain; it also affects other parts of your body.
Even if you don’t normally drink coffee, chances are that you at least understand that it contains caffeine and that caffeine has a naturally stimulating effect on the body. This has long been understood by scientists and consumers, alike.
Testing mice by giving them varying doses of pure caffeine in their water (No coffee for mice!) and then watching them run around their little cages, being more alert, productive, and better looking than the mice that didn’t get the caffeine kept researchers in grant money for decades. But it’s only been since the 1980s that they understood how it actually works.
Caffeine stimulates the nervous system by disrupting it. Without getting into organic chemistry, the blood-brain barrier, and neurotransmitters, suffice it to say that caffeine gets you going by inserting itself into the normal chemistry happening in your brain, and then blocks individual parts of it.
This explains how caffeine stimulates you, but why does it sometimes seem to make you sleepy, as well? That question has several possible answers, and depending on your age, personal physiology, and how much coffee you are drinking (and when you’re drinking it), can be complicated to form a definitive response to.
Vasoconstrictors, Diuretics, and Dehydration… Oh My
Caffeine doesn’t just affect your brain; it also affects other parts of your body. Caffeine’s stimulating effect is system-wide and continues as long as your liver and kidneys haven’t gotten it completely scrubbed from your system.
One of the effects of coffee is the constriction of blood vessels. Another effect is the stimulation of your liver and kidneys to go into overdrive to get rid of it. This is why coffee makes you have to urinate so frequently, and why drinking it can result in dehydration.
If you combine these factors – dehydration and vasoconstriction – you get a situation where your body is trying to push thicker and thicker blood through smaller and smaller blood vessels. This can make you feel groggy, sleepy even.
A Body Out of Balance Tends To…
Because caffeine’s stimulating effects on the nervous system are viewed by your body as a bit of disruption from the normal state of your brain chemistry, your body naturally wants to compensate for these effects. As a result, your body goes into hyper drive, manufacturing adenosine (the neurotransmitter that caffeine disrupts) in an effort to return balance to your body and mind. This effect is responsible for the crash you feel as caffeine’s effects dissipate.
Attempting to Reset Your Clock May Be Messing With Your Rhythm
Another phenomenon that may contribute to the feeling that coffee makes you sleepy is the disruption of your natural rhythm that coffee represents. Human beings have evolved a certain circadian rhythm, a cycle of alertness and rest that kept our species alive in the good old days when we were prey for the big game that we eventually came to hunt.
Not all humans have the same cycle, or even the same length of cycle. Forcing yours to reset every morning by pouring coffee on it can be contributing to your overall sense of drowsiness, and the feeling that coffee is making you sleepy.