Rare cells with widespread connectivity seem to trigger sleep. For a long time, sleep research treated the cerebral cortex as a passive follower. "Usually, sleep is associated with being controlled by subcortical regions," said Geoffrey Terral, a neuroscientist at the Albert Einstein College of Medicine in New York.

In a recent Nature study, Terral and Renata Batista-Brito report a population of cortical cells that challenges that assumption. These cortical cells make up only around 1 percent of the cortex's inhibitory neurons, and switching them on in a mouse puts the animal to sleep. "What our work shows is that the cortex can not only see this rhythm but also initiate it by itself," Terral said.

The cells are called Sst-Chodl neurons. Long-range inhibitory neurons marked by these genes have been known, but targeting them was hard. "A single gene is not able to target these cells," Batista-Brito said. Sst-Chodl cells amount to roughly one neuron in a thousand in the cortex.

"Going after these cells was really a high-risk project," Batista-Brito said. "I wrote a bunch of grants that were always rejected." Her first look proved the risk worth taking. "At first, I saw two or three cell bodies in the whole brain," she said. "Despite that, there was massive arborization all over the visual cortex."