Caffeine Withdrawal
Primarily delivered through beverages like coffee, tea, and soda, caffeine is by far the world's most-consumed psychoactive substance.
Caffeine is chemically similar enough adenosine—a substance we generate naturally over the course of the day inside our bodies—that it temporarily blocks our adenosine-receptors.
Those receptors moderate aspects of our autonomic nervous system, in particular those regulating drowsiness. So when they're blocked, adenosine doesn't register on those receptors, and we feel more alert and awake than we would otherwise based on the quantity of adenosine that has accumulated in our bodies.
This is part of why sleep disruption is a common consequence of consuming caffeine too late in the day: it can take upwards of 12 hours for it to completely unblock those receptors, which prevents our autonomic nervous system from shutting down for sleep the way it typically would.
That drowsiness-killing property is part of what makes caffeine such a popular substance, but it's also been shown to aid focus, reduce fatigue, improve reaction time and motor coordination, and to provide benefits related to cognitive task switching (our ability to shift our attention from one thing to another without suffering from cognitive disruption in the process).
Caffeine has also been shown to provide physical benefits at some doses and in some contexts, ranging from boosts to physical performance and endurance to increases in the consumer's basal metabolic rate and muscle strength. It can also decrease the user’s perception of physical exertion, which means although we're not any less worn out, we may feel less worn out when under the influence of caffeine.
There are quite a few downsides associated with caffeine as well, though, including its capacity to kill those who consume too much of it—though a deadly dose is quite high, and it's unlikely that the casual consumer of caffeinated beverages (as opposed to concentrated caffeine powders or similar products) would ever even accidentally reach it.
Most of the adverse consequences of caffeine are instead related to jitteriness, anxiety, sleep issues, digestive and gastric acid secretion problems, and a chemically reinforced type of dependence that—while not as severe as the chemical dependence caused by true, clinical addition—can be practically the same because of the nature of caffeine and how it makes users feel.
Over time, our bodies become acclimated to caffeine, which reduces many of its most prominent effects: it'll likely still help a bit with drowsiness, for instance, but not with the same potency as when we first started consuming it.
If we stop consuming caffeine after having it every day for months or years, our bodies—having become accustomed to its presence—can go a little haywire, which we experience as withdrawal symptoms that include headaches, dizziness, tiredness, brain fog, body aches, insomnia, a mild depressed feeling, and even cardiac issues or hypertension.
As of the 5th edition of the Diagnostics and Statistical Manual of Mental Disorders (the DSM-5), caffeine withdrawal is an officially recognized diagnosis; which is wild, considering that something like 85-90% of the global human population consumes at least one caffeinated product every single day (96% of those caffeinated products being coffee, tea, or soft drinks/sodas/fizzy drinks) and are thus prone to falling into this diagnosable category if they skip this habit for just a day or two.
One of caffeine's main benefits, then, is that it helps prevent the very withdrawal symptoms it can cause: it's a self-reinforcing consumption cycle.
On top of the ritualistic side of caffeine-related habits, then, and the physical and cognitive benefits it provides, part caffeine's appeal is that it helps us avoid a clinical disorder that emerges during the week or two it can take our bodies to return to their baseline, non-caffeinated state.
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