"Drinking Coffee Right After Arriving at Work Is Risky... Best to Drink Starting from 'This Time'"

"Best to Drink Between 9:30 and 11:00"
Too Early Coffee Is a Source of Stress

A claim has emerged that it is best to drink the first coffee of the morning at least after 9:30 AM.

According to interviews with The Wall Street Journal and others, Gregory Marcus, a cardiologist at the University of California, San Francisco, explained the reason for this.

A citizen is walking while holding a coffee.
Photo by Yonhap News

According to Professor Gregory, when our body wakes up from sleep, the secretion of cortisol, one of the stress hormones, increases. Cortisol is highest immediately after waking up in the morning and then gradually decreases. However, drinking coffee when cortisol levels are high causes cortisol levels to continue to rise.

The time when cortisol levels in our body drop the most is between 9:30 AM and 1:00 PM. Therefore, Professor Gregory said that it is best to drink the first coffee of the day between 9:30 AM and 11:00 AM.

While the first coffee is important, the last coffee is also crucial. Professor Gregory stated that the last coffee of the day should be consumed at least six hours before going to bed.

Caffeine interferes with deep sleep that restores bodily functions, and drinking coffee too late can cause caffeine’s stimulating effects to disrupt sleep. According to the U.S. Food and Drug Administration (FDA), the sleep-disrupting effects of caffeine diminish 4 to 6 hours after caffeine intake.

Professor Gregory explained, "The impact of caffeine on sleep depends on how quickly caffeine is metabolized," adding, "Due to genetic differences, people’s caffeine metabolism rates can be fast or slow."

A study published in the New England Journal of Medicine (NEJM) found that people with slow metabolism sleep about one hour less after drinking coffee, whereas those with fast metabolism show little change in sleep activity.

The FDA recommends an adult daily caffeine intake of 400 mg or less, which is about 3 to 4 cups of coffee. Additionally, the French National Institute of Health and Medical Research stated that a moderate caffeine intake that invigorates health is 100 to 150 mg per day.