Some people who function normally on just 6 hours of sleep harbor an altered version of a particular gene, the second gene so far linked to short sleep, according to new research.

In 2009, researchers described a mother and daughter with a mutation in a gene called DEC2 who felt well rested after with about 6 hours of sleep per night. (Many experts recommend that adults get at least 7 hours.) DEC2 codes for a protein that helps turn off the expression of other genes, including the gene for the hormone orexin, known to regulate wakefulness.

Now, by studying another family containing naturally short sleepers, the scientists have identified another mutation, which they estimate is present in roughly four of every 100,000 people. Mice genetically engineered to have this mutation slept, on average, 1 hour less per day than controls, the researchers report online today in Neuron.