Why Does Coffee Energize Some People While Having No Effect on Others?

“Coffee doesn’t really affect me,” said 20-year-old Emily as she left a café in central London. “I usually drink two cups of coffee a day and maybe an energy drink as well. Even if I have Coca-Cola all night, I still feel fine.” We all know friends like Emily who can have a double shot espresso or an energy drink and still fall asleep easily within a few hours. However, for many others, the effects of coffee linger much longer. So, why can some people drink coffee before bed and sleep peacefully while others stay restless all night?

Caffeine is the world’s most widely used psychoactive substance. Over two billion cups of coffee are consumed globally every day. Additionally, caffeine is found in cola drinks, energy beverages, certain chocolates, chewing gum, and sports supplements, making its use incredibly widespread. Caffeine helps us stay alert, improves concentration, and reduces feelings of fatigue. However, excessive intake can cause headaches, restlessness, increased heart rate, and difficulty sleeping at night.

According to Dr. Marilyn Cornelis, Assistant Professor of Nutrition at Northwestern University in Chicago, caffeine can take anywhere from 6 to 20 hours to be fully eliminated from the body, varying greatly among individuals. “You may not feel the effects immediately after your first cup of coffee,” she explained. “But caffeine remains in your system for two to three hours, and drinking a second cup adds more caffeine, which can amplify its effects and cause anxiety.”

Sleep researchers recommend avoiding caffeine at least six hours before bedtime. The United Kingdom’s National Health Service (NHS) advises pregnant women to consume less than 200 milligrams of caffeine per day — roughly equivalent to two cups of filtered coffee. For coffee enthusiasts, there is some good news: regularly drinking coffee in the mornings is linked to better health.