Even sipping Coke or decaffeinated coffee can make it difficult to fall asleep. Why is that? That’s because the total amount of caffeine consumed per day affects sleep. Even decaffeinated coffee contains a little bit of caffeine. But it’s only 5-10 mg per cup. We should drink about 10 cups of decaffeinated coffee to expect its caffeine effect.  In coffee, substances other than caffeine either lack the pharmacological activity to keep us awake or are negligible in quantity. However, I heard some people saying they could not fall asleep after they drank decaffeinated coffee. The problem lies in the cumulative intake of caffeine rather than the decaffeinated coffee itself. Drinking coffee in the morning and decaffeinated coffee in the afternoon can interfere with sleep.

Coke also contains caffeine. But compared to coffee, it's a small amount. A 1.5 liter-bottle of Coke contains the same amount of caffeine as one cup (355 ml) of Starbucks iced Americano. But some people lose sleep after drinking coke. This is because of the timing of drinking coke. Unlike coffee, people find it casual to drink coke after dinner. People drink coke during the meal when having pizza or fried chicken. However, the caffeine in the coffee you drank in the morning and at noon does not disappear from your body just in the evening. It usually takes four to six hours for the caffeine level in the blood to drop by half. Half of the caffeine from the coffee you drank at noon remains until 6 p.m. That means if a cup of coffee has 150 mg of caffeine, 75 mg will still be in your system at 6 p.m. At this time, if you drink 500 ml of coke again, 50-60mg of caffeine is added. This is an amount that can have an effect similar to drinking your first cup of coffee at 6 p.m.

As we get older, we may become more sensitive to caffeine. A person who didn't mind drinking five or six cups a day in school days becomes middle-aged and suffers from insomnia when drinking two or three cups of coffee a day. This is because the liver metabolizes caffeine more slowly when we age. There are individual differences in caffeine metabolism. Some middle-aged people can drink three to four cups of coffee with no trouble. Smokers, regardless of age, tend to drink a lot of coffee. Detoxification enzymes in the liver work harder to quickly clean and send out harmful substances in cigarette smoke and caffeine is also cleaned faster in this process. If you quit smoking or switch to an e-cigarette from traditional cigarettes, the drug-metabolizing enzyme activity also returns to the normal level. After quitting smoking, you should reduce the number of cups of coffee per day to avoid excessive caffeine.

Can’t fall asleep even though you had a very low consumption of caffeine all day? Insomnia is caused by many different things. The mere thought of having to wake up early the next morning can make it difficult to sleep. If symptoms get severe or persist, consult a specialist. But counting the total amount of caffeine consumption per day can greatly help improve the quality of sleep. 

Jeong Jae-hoon is a food writer and pharmacist. He covers a variety of subjects, including trends in food, wellness and medications. This column was originally published in Korean in Joongang Ilbo on March 1, 2023. – Ed.