What drinking coffee at night does to your body clock

A small and preliminary study suggests that caffeine does more than serve as an eye-opener: When consumed a few hours before bed, the most widely consumed psychoactive drug in the world seems to disrupt the body’s internal clock.

And this could cause jet lag-style sluggishness during daylight hours, the study authors suggest.

The research doesn’t say anything about how coffee consumption in the morning or throughout the day may affect the body’s internal clock. And the findings need to be confirmed.

Still, it seems likely that coffee at night “isn’t just keeping you awake,” said study co-author and sleep researcher Kenneth Wright Jr., a professor with the Department of Integrative Physiology at the University of Colorado at Boulder. “It’s also pushing your [internal] clock later so you want to go to sleep later.”

At issue: The body’s circadian clock, which sets biological rhythms such as sleep/wake cycles. Every cell in the human body has a clock, Wright said.

The new study aims to understand how caffeine may affect the body clock. Other research has suggested that caffeine disrupts body clocks in other organisms and species such as algae, fruit flies and perhaps mice, he said.

Wright and his colleagues examined five people who were studied over 49 days. Three hours before their regular bedtime, they were assigned to consume a capsule of caffeine equal to a double espresso — with the amount adjusted to their body size — or a placebo capsule. They were also exposed to either bright or dim light. Bright light can reset the body clock and make people want to go to bed later.

The researchers found that the caffeine appeared to delay the body clocks of the study participants by 40 minutes, about half the delay linked to exposure to bright light.

The amount of caffeine was small, the equivalent of about a double espresso or medium cup of coffee for most people, Wright said. “We’re not talking about a lot of caffeine here.”

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