Caffeine, the main active ingredient in coffee, has a deserved reputation as an energy booster. But caffeine is also a drug, which means it can affect us all differently, depending on our consumption habits and our genes.
“The paradox of caffeine is that it helps with attention and alertness in the short term. It helps with some cognitive tasks and it helps with energy levels,” said Mark Stein, a professor in the Department of Psychiatry and Behavioral Sciences at the University of Washington who has studied the impact of caffeine on people with ADHD. “But the cumulative effect — or the long-term impact – has the opposite effect.”
Some of caffeine’s paradoxical effects stem from its effects on what researchers call “sleep pressure,” which makes us increasingly sleepy as the day goes on. From the moment we wake up, our body has a biological clock that prompts us to go back to sleep later in the day.
Seth Blackshaw, a neuroscientist at Johns Hopkins University who studies sleep, said researchers are still learning how sleep pressure builds up in the body, but that over the course of the day, our cells and tissues use and burn energy in the form of a molecule called adenosine triphosphate or ATP. As that ATP is consumed—when we think, exercise, run errands, or meet—our cells generate a chemical called adenosine as a byproduct. That adenosine will bind to receptors in the brain, making us sleepier.
Chemically, caffeine resembles adenosine enough on a molecular level that it occupies those binding sites, preventing adenosine from binding to those brain receptors. As a result, caffeine works to temporarily suppress sleep pressure, making us feel more awake. Meanwhile, adenosine continues to accumulate in the body.
“Once the caffeine wears off, you get a very high sleep pressure and you have to pay that back,” said Dr. Bradshaw. In fact, sleep is the only way to relieve and reset an increased level of sleep pressure.
The problem is that the more we drink caffeine, the more we build up our body’s tolerance for it. Our liver adapts by making proteins that break down caffeine more quickly, and the adenosine receptors in our brains multiply so they can remain sensitive to adenosine levels to regulate our sleep cycle.
Tired of tossing and turning? There are some strategies you can try to improve your hours in bed.
Ultimately, sustained or increased caffeine consumption negatively impacts sleep, also making us feel more fatigued, said Dr. stone.
“If you’re sleeping less and stressed, and you rely on caffeine to improve it, it’s just a perfect storm for a short-term fix that will make things much worse in the long run,” he said. “You’re going to add more shots to your espresso, but the negative impact on your sleep will continue, and that’s cumulative.”
Caffeine can also cause spikes in blood sugar or lead to dehydration — both can make us more tired, said Christina Pierpaoli Parker, a clinical researcher who studies sleep at the University of Alabama at Birmingham.
If you feel an afternoon slump even after a cup of coffee, the solution may be to consume less of it, scientists say. Don’t drink it every day, or go cold turkey for a few days so your body can clear all the caffeine from your system, then gradually add it back into your routine. Ideally, drinking coffee should be “fun and beneficial, and really give you a boost when you need it,” said Dr. Bradshaw.
In the meantime, if you feel like caffeine is no longer giving you an energy spurt, experts recommend taking a nap, exercising, or sitting outside and getting some exposure to natural light, which of course gives a boost of can give energy.
“Watch your sleep and make sure you get a good night’s sleep,” said Dr. stone. “Sufficient sleep and exercise are the first-line interventions for attention problems and sleepiness. Caffeine is a useful addition, but you don’t want to become dependent on it.”
Wudan Yan is an independent journalist in Seattle who covers science and society.