Coffee is without a doubt one of the most popular nonalcoholic beverages in the country. Many people enjoy the taste, but most people drink the beverage for its caffeine content, which makes you feel alert and upbeat. According to a report from Slate, however, caffeine isn’t all good. A new study reveals that it can seriously throw your circadian rhythms, or the internal clocks that regulate sleep.

Caffeine has been shown to alter the circadian rhythms of red bread mold, fruit flies, green algae, and sea snails. The most recent study, published this Wednesday in Science Translational Medicine, measured how caffeine affected the circadian rhythms of five coffee drinkers over a 49-day period.

One of the other main influencing factors in circadian rhythms is light. Each participant sat through four phases of different circadian combinations: dim light, dim light with caffeine, bright light, and bright light with caffeine.

The participants ate either a placebo or a caffeine capsule, equivalent to a double espresso, about three hours before they went to sleep. Researchers measured changes in melatonin in each participant to determine how the caffeine affected the body’s internal clock.

The researchers discovered a “physiological meaningful” effect that was quite pronounced in the participants who consumed the caffeine pill. On average, caffeine users saw their circadian rhythms begin to shift 40 minutes later than those who took a placebo. This is about half of the magnitude of change caused by bright light.

Sleep isn’t the only thing regulated on a circadian rhythm. The researchers also found that caffeine also affected internal organs and muscles.