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.