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Why we're sleepy after meals
Scripps Howard News Service


June 04, 2006

That after-meal desire for a nap is as natural for humans as it is for lions or lap dogs.

Scientists have pinpointed for the first time how the sugar in food turns off the brain cells that keep us awake and makes us crave a siesta after a big meal.

"It has been known for a while that people and animals can become sleepy and less active after a meal, but the brain signals responsible for this were poorly understood," said Dennis Burdakov, a researcher at the University of Manchester in England who led the study published this week in the journal Neuron.

Working with specially engineered mice, Burdakov's team demonstrated exactly how glucose blocks or "inhibits" the brain cells that make orexins, tiny proteins that regulate our state of consciousness.

"These cells are critical for responding to the ever-changing body energy state with finely orchestrated changes in arousal, food seeking, hormone release and metabolic rate to ensure that the brain always has adequate glucose," the researchers wrote.

By genetically manipulating the mice to produce a florescent protein that lights up only in the orexin cells, the researchers could study how glucose reacts in those neurons.

Specifically, they "identified the pore in the membrane of orexin-producing cells that is responsible for the inhibiting effect of glucose,'' Burdakov said.

"This previously unknown mechanism is so sensitive, it can detect minute changes in glucose levels, the type that occurs between meals, for example. This may well provide an explanation for after-meal tiredness and why it is difficult to sleep when hungry."

Malfunction of the orexin brain cells can cause narcolepsy, when a person can't stay awake, and is tied to obesity. There is also evidence that the same brain cells play a role in learning, reward-seeking and addictions.

"Now that we know how glucose stops orexin neurons 'firing,' we have a better understanding of what may occur in disorders of sleep and body weight."


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