How does caffeine work?

“The stimulant effects that most people associate with caffeine are due to caffeine’s ability to block adenosine receptors in the brain,” explains Marilyn Cornelis, associate professor of preventive medicine at the Feinberg School of Medicine at Northwestern University.

Adenosine, a natural sedative, builds up during waking hours and dissipates during sleep.

But when caffeine blocks the adenosine receptors, adenosine can’t do its job…so you don’t feel drowsy.

Caffeine (C) binds to adenosine receptors in the brain. That prevents adenosine (A), which is a natural sedative, from attaching itself and making you feel drowsy.

Like with most dependencies, you can build up a tolerance to caffeine.

“The more caffeine you consume, the more adenosine receptors your brain makes,” notes Cornelis. So you need even more caffeine to block those extra receptors and keep you alert.

How much is too much?

The Food and Drug Administration says that most adults can safely consume up to 400 milligrams a day.

But caffeine’s impact on people varies.

“On average, the half-life of caffeine— which is the amount of time that it takes to break down half the caffeine in your body—is about three to five hours,” says Cornelis. So if you drink a cup of coffee at, say, 2 p.m., a quarter of its caffeine may still be coursing through you between 8 p.m. and midnight.

“But there’s large genetic variability in the activity of the key enzyme that metabolizes caffeine,” says Cornelis.

What’s more, caffeine metabolism can change. For example, it speeds up if you smoke cigarettes and slows if you take birth control pills or are pregnant.

Of course, you don’t need a genetic test to find out how caffeine affects you.

“Those tests only look at the gene for metabolizing caffeine,” Cornelis points out. “But maybe you have a genetic variant in the adenosine receptor, which could make you more or less sensitive to caffeine’s effects on sleep. The test won’t tell you that.”

What’s more, most people know how much caffeine they can handle.

“People with the genetic variant that is linked to slower caffeine metabolism generally consume less caffeine,” says Cornelis. “If you drink one cup of coffee and you’re off the wall for the rest of the day, that’s more informative than a genetic test.”

Caffeine & sleep

“I’m sleeping like a teenager again and wake feeling actually refreshed,” says Michael Pollan about his breakup with caffeine in his audiobook Caffeine.

But Pollan’s experience may not apply to everyone.

In one of the few studies to look, researchers had 66 young regular caffeine users who had trouble sleeping go cold turkey. Over the next week, the volunteers spent no more time asleep and took no less time to fall asleep than before.

Even so, it may be wise to avoid high doses of caffeine later in the day.

In an industry-funded study, re­searchers gave 12 young regular caf­feine users either a placebo or 400 mg of caffeine six hours, three hours, or immediately before bedtime on separate days. Compared to when they got the placebo, the volunteers slept about an hour less no matter when they got the caffeine.

Studies show that people take longer to fall asleep, get less deep sleep, and wake up more often throughout the night when they’re given caffeine right before bed. (There’s little research on whether having caffeine in the morning or early afternoon also impairs sleep.)

The bottom line

If you think caffeine is keeping you from getting a good night’s rest, try cutting back or cutting it out after midday.

Photo: boyloso/ Illustration: Jorge Bach/CSPI.