Caffeine within a few hours of hitting the sack can wire your cells to stay awake, says a new sleep study on circadian rhythm

By Charlotte Hilton Andersen
Updated: October 14, 2015
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You probably haven't heard, but coffee wakes you up. Oh, and caffeine too late in the day can mess with your sleep. But a new, less obvious study has revealed exactly how coffee affects your daily rhythms, and it may be costing you more z's than you think. Caffeine may actually alter your circadian rhythm, the internal clock that keeps you on a 24-hour sleep-wake cycle, according to research in Science Translational Medicine.

Each cell in your body has it's own circadian clock and caffeine disrupts a "core component" of it, said study Kenneth Wright Jr., Ph.D., co-author of the paper and a sleep researcher at the University of Colorado at Boulder. "[Coffee at night] isn't just keeping you awake," Wright explained. "It's also pushing your [internal] clock later so you want to go to sleep later." (It's likely one of 9 Reasons You Can't Sleep.)

How much later? A single serving of caffeine within three hours of bed pushes back your sleepy time by 40 minutes. But if you buy that coffee in a a well-lit coffeeshop, the combo of artificial lighting and caffeine can keep you up almost two extra hours. This jives with a 2013 study in the Journal of Clinical Sleep Medicine that found that just one coffee affects your sleep up to six hours after drinking it.

But this news that caffeine can alter your circadian rhythms may have wider consequences, as your internal clock controls a lot more than just your sleep. In fact, it influences everything from your hormones to your cognitive abilities to your workouts, messing it up can throw your whole life off.

Wright advised removing coffee from your diet or just having it in the morning if you are having problems sleeping at night. (The 2013 study advised having caffeine no later than 4 p.m. if you are aiming for a 10 p.m. bedtime.) But, Wright added, the study was quite small (just five people!) and caffeine effects everyone differently, so the best study to rely on may be the one you do on yourself.



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