The other day during a presentation on stress and sleep, a man in the audience asked me a question about coffee. How much is too much? he wanted to know.
“I sleep about an hour less than I used to,” he explained. “Could coffee have something to do with it?” He added that the cups he normally gets are about twice the size of normal cups. “I drink 4 or 5 big cups.”
I suggested that he cut back on coffee and left it at that. Yes, the number one symptom of excessive coffee consumption is insomnia, and some doctors still urge insomnia sufferers to go cold turkey. But research now suggests that people vary in their sensitivity to caffeine. What’s too much coffee for one person may not be too much for another.
Caffeine and Your Health
In moderation, caffeine isn’t bad for your health. The Mayo Clinic and other trusted medical authorities claim that up to 400 mg of caffeine—or about 4 cups of brewed coffee—is safe for most adults.
In fact, moderate amounts of caffeine may have protective effects, including lowering the risk of developing melanoma, diabetes, and Alzheimer’s and Parkinson’s diseases. A meta-analysis published by the American Heart Association in 2012 concluded that “moderate coffee consumption is inversely associated with risk of heart failure, with the largest inverse association observed for consumption of 4 servings per day.”
Other research suggests that heavier coffee drinking comes with costs. In one study, all-cause mortality was higher in heavy coffee drinkers 54 years and younger than in their counterparts who drank less coffee, leading to the recommendation that young and middle-aged adults avoid drinking more than 4 cups of coffee a day. But in another study, investigators found no relationship between caffeine consumption and all-cause mortality.
Coffee interferes with secretion of the neurotransmitter adenosine: this is what pumps you up and masks sleepiness. But some people are caffeine sensitive and others aren’t—this has largely to do with your genetic make-up. Depending on which variant of the adenosine receptor A2A gene you have, drinking coffee in the evening may degrade your sleep quality and decrease deep sleep—or have no effect at all. Coffee may improve your performance when you’re feeling sleep deprived—or you’ll get no boost from caffeine at all.
Whether and how much caffeine will affect your sleep also depends to some extent on circadian variables and the time of day when you feel most alert. Stanford researchers found that salivary concentrations of caffeine during sleep strongly correlated with wakefulness at night in early risers; less so in intermediate types; and not significantly in night owls. So particularly if you’re an early riser, you’ll need to be careful of when you drink caffeinated beverages and how much you drink.
Metabolism and Elimination
People also differ greatly in the speed at which their bodies metabolize and eliminate caffeine and its metabolites. About 30 to 60 minutes after consumption is the time of maximum plasma concentration. But some people reportedly reach this point in 15 minutes. Metabolism occurs in the liver by an enzyme called CYP1A2. Slight variation in the gene controlling this enzyme is the main determinant of how quickly processing will occur.
Caffeine has an average elimination half-life of 5 to 6 hours. So if you’re like the average person (whatever that is!), the amount of caffeine in your blood will have decreased by half in 5 or 6 hours.
In reality, though, caffeine’s half-life can vary from 2 to 12 hours, depending on constitutional as well as extrinsic factors. Smokers, for instance, metabolize caffeine almost twice as fast as nonsmokers. Pregnant women, in contrast, clear and excrete caffeine more slowly than women who are not pregnant. And as we age, we metabolize all drugs more slowly, including caffeine.
So how much caffeine is too much? Differences in the way people process caffeine make it hard to make a one-size-fits-all pronouncement. If you’ve got insomnia, try cutting back on caffeine gradually and make sure you’re not indulging too late in the day.