When you’re tired, it’s common to pour a cup of coffee or take a power nap, but research shows that combining the two is more effective than simply doing one or the other.

The “coffee nap,” as it’s been called, works because of how caffeine and sleep interact with a molecule called adenosine.

When you’re awake, neurons in the brain produce adenosine, a byproduct of the cell activity, and the buildup of this molecule contributes to feelings of tiredness.

But when you consume caffeine, it passes into the bloodstream and eventually reaches the brain where, because of its structural similarity to adenosine, it fits into adenosine receptors and blocks some of them.

As Stephen R. Braun writes in "Buzz: the Science and Lore of Alcohol and Caffeine," it's like "putting a block of wood under one of the brain’s primary brake pedals."

But here’s what makes the coffee nap more effective than caffeine alone: Sleep naturally removes adenosine from the brain, so a 20-minute nap can perk you right up. It also takes about 20 minutes for caffeine to work its way into your bloodstream, so following a cup of coffee with a power nap will reduce adenosine levels just as the caffeine kicks in.

While scientists haven’t actually observed the effects of a coffee nap on the brain, research shows that caffeine and a nap maximizes alertness.

In a series of studies at Loughborough University, participants who took coffee naps committed fewer errors in a driving simulation than when they drank only coffee or only took a nap.

A study at Hiroshima University found that participants who drank caffeine and took a short nap performed significantly better than people who took other measures to stay alert, including taking a nap, washing their faces or having a bright line shone in their eyes.

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