Coffee has been celebrated for centuries as a not-so-secret weapon in the pursuit of greater mental acuity. Many of us feel cloudy headed in the morning before our first cup and can attest to the efficacy of an afternoon mug to get us through the “two o’clock doldrums.” And there’s plenty of research to indicate that, indeed, caffeinated coffee gives us a short-term burst of increased alertness and responsiveness. Yet for all of coffee’s cognitive benefits, an overreliance on it can backfire and leave us feeling even more sluggish than without it. That especially tends to be true for those who push their coffee consumption later and later into the day and those .

How coffee helps

Coffee is one of the most studied substances consumed by humans, and the preponderance of results suggests that the health impacts of coffee are largely beneficial, with minimal health risks. For example, a recent study from China found that people who drink two to three cups of coffee and two to three cups of tea each day had a stroke risk 32% lower than non-tea or -coffee drinkers and were also 28% less likely to develop dementia. In other research, women and men who drank three cups of coffee each day showed less risk of developing cognitive decline or memory problems than non-drinkers. Other studies have found that coffee consumption contributes to increased longevity and confers protection against coronary heart disease, heart failure, and heart arrhythmias.

Coffee’s long-term cognitive benefits may stem from the antioxidant and anti-inflammatory properties of its more than 1,000 bioactive compounds such as flavonoids. Many diseases, including cognitive decline and dementia, are recognized to have inflammation as a contributing factor, and coffee may help stave them off by reducing oxidative stress and inflammation in the brain and throughout the body. Compounds in coffee may also boost metabolism and reduce the absorption of fat in the gut, thus reducing the likelihood that obesity and insulin resistance will contribute to deteriorating health. And in a 2023 study conducted by researchers at the University of California, San Francisco, volunteers who wore sensors tracking their biological activity walked an average of 1,000 extra steps on the days when they drank coffee. Over the course of a lifetime, that extra exercise could have a cumulative effect in protecting coffee drinkers’ hearts.

In the short term, coffee gives us a mental boost because one of its key components is caffeine, a stimulant. When we drink coffee, our heart rates increase and more blood reaches our brains. Caffeine has an almost immediately felt effect on our central nervous systems, increasing energy metabolism in our brains, which makes us feel alert and sharp.

The downside

As helpful as coffee may be both for a short-term mental boost and long-term brain health, it can become a trap for some people. For one thing, coffee is decidedly habit-forming, and it’s easy to begin to feel reliant upon it for daily functioning. That may be just fine for some people, while it doesn’t sit right with others.

Our bodies behave according to cyclical rhythms, and it’s common to experience an afternoon slump. Many of us go straight to the coffee pot when that happens, depending on our old friend Joe to help us power through the remainder of the day. When we find that that works, it’s easy to assume that a second or third cup will be even better, and we’re tempted to extend our coffee-drinking on into the evening. But the effects of a cup of coffee can be felt several hours later, which means that your brain is still in a state of stimulation when you’d rather be asleep. Put in a restless night, wake up groggy in the morning, gulp down lots of coffee to get through your day, sleep poorly again, and now you’ve entered a negative cycle in which coffee has ceased to be a net positive for your cognition.

In the same study referenced above in which people took more steps when they drank coffee, they also slept an average of 36 fewer minutes per night on the days when they drank coffee. Given the importance of sleep not just for mental alertness but for overall health, the exercise benefit may be cancelled out by the sleep deficiency.

Idiosyncratic effects

Despite tendencies toward common patterns, there’s a lot of individual variation when it comes to caffeine and sleep. You’ve probably known people who feel jittery all day after a single cup of mild coffee in the morning and others who can gulp down a mug at 8:00 p.m. and still sleep just fine. Such anecdotal observations are borne out by research that finds caffeine tolerance to be a highly idiosyncratic thing. That’s why it’s difficult to frame recommendations in terms of, “No coffee X amount of hours before bedtime.” But certainly if you’re having trouble sleeping, reducing your caffeine consumption, especially in the afternoon and evening, is a smart place to start.

Note, too, that although caffeine before bedtime might not keep your brain awake, it stimulates both the bowel and bladder. So you might have to reawaken during the night to use the bathroom, which of course disrupts the restful slumber you need to keep your brain quick and healthy.

Best practices

All told, coffee should be viewed as a net neutral when it comes to cognitive functioning and health. If you enjoy it, there’s no reason to stop, as long as you’re mindful of its effects on your sleep cycles. Just keep these things in mind:

  • Cut the sugar. Learn to enjoy coffee without adding sweeteners, which undo many of coffee’s health benefits.
  • Don’t overdo it. Most research linking coffee consumption to health and cognitive benefits finds one to three daily cups to be the sweet spot.
  • Try other solutions to the afternoon doldrums. To avoid falling into a cycle of coffee dependence, try rehydrating with water, taking a quick walk or doing some deep breathing exercises. Just a few minutes can help you conquer the slump without resorting to caffeine.
  • Learn your limits. Most people can figure out just fine where their afternoon cutoff is for consuming caffeine. Once you’ve established that you won’t be able to sleep if you have coffee after 2:00 p.m., for example, create a hard rule for yourself and stick to it.
  • Make room for decaf. Coffee can be a wonderfully social drink. If you’re invited out for coffee but have already had your daily allotment of caffeine, order a decaffeinated cup. Cutting your coffee with decaf is also a great way to begin to reduce your dependence on caffeine. Just be aware that even decaffeinated coffee contains some caffeine (about 3% the amount contained in regular coffee), so if you’re particularly sensitive to caffeine, you still might disrupt your sleep by consuming it too late in the day.

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