Siesta Science: Why Lunch Makes Us Sleepy

The afternoon siesta is not just a cultural tradition in some countries. It's a biological reaction to lunch.

Researchers have revealed how the sugar in food, called glucose, "can stop brain cells from producing signals that keep us awake," said Denis Burdakov of the University of Manchester in England.

"It has been known for a while that people and animals can become sleepy and less active after a meal, but brain signals responsible for this were poorly understood," Burdakov said. His team figured out how glucose blocks neurons that make orexins, which are tiny proteins that help us stay conscious .

"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," Burdakov explained today. "We have identified the pore in the membrane of orexin-producing cells that is responsible for the inhibiting effect of glucose."

Malfunction of orexin neurons can lead to narcolepsy  — the chronic inability to stay awake — and obesity.

"Now we know how glucose stops orexin neurons 'firing', we have a better understanding of what may occur in disorders of sleep and body weight," Burdakov said. "This may well provide an explanation for after-meal tiredness and why it is difficult to sleep when hungry."

The study is detailed this week in the journal Neuron.