Why An Evening Coffee Is Worse Than You Think

It can be so tempting to grab a cup of coffee after dark, especially as the weather turns chillier. But new research suggests that evening caffeine disturbs your internal clock even more than you think, so it's time to put down today's second pumpkin spice latte and try a different alertness strategy instead.

Researchers from several universities decided to test the real effects of caffeine on sleep by taking some experimental participants and locking them away from the light of day for over a month and a half. Some of the participants were given caffeine doses (about the same as a double espresso) three hours before bed, and others were given a placebo.

The researchers' results, published in the journal of Science Translational Medicine, show that they were able to lengthen the biological day of people who consumed caffeine before bedtime. Their circadian rhythms became delayed by about 40 minutes when they consumed the caffeine three hours before bed, due to a later melatonin (the sleepiness-inducing hormone) release. For comparison, this effect was about twice as strong as the effect of being exposed to bright light before bedtime.

These findings are great news for jet-lagged travelers, who need a way to align themselves with local time quickly. But it's bad news for everyone else who's just looking for a way to feel quite awake until it's bedtime, then to feel quite sleepy. Unfortunately, our bodies just aren't really capable of snapping on and then snapping off at will.

Forty minutes may not seem like that big of a deal, but it can easily cascade into a really rough night — and a really rough next day. Let's say you usually feel tired around 10 p.m., but you have a 7 p.m. caffeinated beverage. It buys you another hour or so of productive time, but now you aren't going to feel tired until around 11 p.m. instead. You mistake your burst of energy for a real second wind, but then you end up tinkering around online for a couple hours, and the light from your computer screen further delays your melatonin release. Now you're too tired to get ready for bed, and you delay further, not getting in bed until midnight, sort of wound up but sort of exhausted. Oops.

So, next time someone suggests meeting up in the evening, steer away from the coffee shop (and an alcoholic nightcap isn't great for your sleep either, by the way). Beyond the obvious effects of poor or short sleep, sleep deprivation makes it harder to interpret social cues, which isn't going to do your personal or professional lives any favors. If you always feel tired at night this fall and winter, you might actually be suffering from seasonal affective disorder instead of normal end-of-day fatigue, so be sure that you're treating the source of the problem instead of just covering up its symptoms.

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