Sugar makes mice sleepy

Contrary to parental belief, sugar may actually cause drowsiness, not hyperactivity. Key brain cells awash in glucose put mice to sleep, scientists report July 8 in the Journal of Neuroscience.

“We all experience this strong feeling of sleepiness after a very large meal,” says study coauthor Christophe Varin of Lyon Neuroscience Research Center and ESPCI ParisTech in France. Sugar is the reason those post-meal naps are often irresistible, the results suggest.

This study offers hard evidence that sugar promotes sleep, an idea that had been largely anecdotal, says neuroscientist Denis Burdakov of the Francis Crick Institute in London. “It’s really great to see it measured properly,” he says.

Varin and colleagues injected glucose directly into the brains of mice. Meals, particularly those loaded with carbs, raise sugar levels in the brain, too. This glucose might spur similar brain cells in people to exert sleepy effects.

Scientists already knew that specialized nerve cells in the mouse brain are sleep pushers. These neurons, nestled in the ventrolateral preoptic nucleus, or VLPO, tamp down their sleep-inducing activity in response to various brain messengers that promote wakefulness. The new study shows that glucose has the opposite effect —amping up these sleep-promoting cells.

In the first two hours after receiving glucose injected directly into the VLPO, mice fell into a deep sleep called slow-wave sleep faster and stayed there longer than mice injected with a nonsugar solution, the team found. People and mice spend much of their sleeping time in slow-wave sleep as opposed to more active REM sleep.

VLPO neurons can directly sense the glucose in their neighborhood, other experiments revealed. And the more glucose the neurons detected, the more they fired their sleep-inducing signals.

The idea that sugar can lead to drowsiness may not be popular among energy drink manufacturers. It is possible that such drinks, which are often chock-full of sugar, can lead to immediate surges in alertness. But over longer periods of time, high-sugar drinks, at least those with little caffeine, might actually enhance sleepiness, as scientists reported in Human Psychopharmacology: Clinical and Experimental in 2006. The new study may help explain why that sugar blast made participants less alert.

There may be a good reason to sleep after a high-sugar meal, Burdakov says. Sleeping soon after eating ensures that a person or animal sticks close to a good food source, he says. “Moving less when sugar is around makes basic survival sense.”

Scientists suspect that people have cells similar to the VLPO neurons in mice, but more experiments are needed to reveal just how those cells might respond to sugar.