Is marijuana now the most widely used psychoactive substance? Nope. And it's not alcohol or nicotine, either. It's actually caffeine, that little gem lurking in your morning cup of coffee. Caffeine is known to be a stimulant; people use it most often to get a little boost of energy. But exactly what is happening in your body when you consume caffeine, either in coffee, tea, or energy drinks, or in foods like chocolate, hasn't been totally figured out yet. Researchers currently think that the caffeine buzz has something to do with the inhibitory neurotransmitter adenosine. When adenosine receptors are normally activated, they tend to cause sedative effects, such as reducing your heart rate and suppressing the release of other neurotransmitters (including glutamate and dopamine). But caffeine may work by binding to (and blocking) adenosine receptors in your brain and throughout your body. And once those adenosine receptors are blocked, the other neurotransmitters get to hang out and excite your system. Fun fact: This same mechanism is thought to play a part in caffeine's addictive properties.
Caffeine messes with our daily rhythms in other ways, too. For one, it affects the way our bodies release cortisol, commonly known as the "stress hormone." Caffeine actually causes a pretty solid increase in cortisol levels in those who aren't normally coffee-drinkers. But in those who enjoy their daily caffeinated beverages — and have thus built up at least a partial tolerance to the chemical — the effects are a little less clear. But we do know they last for a long time. Even cutting off your caffeine six hours before bedtime may not be enough to ensure you doze off easily. And that's important because those aforementioned adenosine receptors also play a role in keeping our circadian rhythms humming along. So, if you're a regular coffee-drinker, you might want to be extra careful about sticking to your routine. An ill-timed coffee break or — even worse — a skipped one, could make it hard to get back into your normal sleep-wake cycle. On top of all that, we know that different people react to coffee (and its caffeine) differently. While it is pretty much a requirement for some, others complain that it makes them far too jittery to get anything done. And researchers are now learning that difference may depend at least partly on your genes. But with all of that power — good and bad — it's no wonder so many of us start our mornings with some form of caffeine.