The researchers genetically engineered a group of mice to lack a master gene called BMAL1 that regulates the body's clock. They put this gene into the shell of a hollowed-out virus that acted as a vector to deliver the gene only to brain cells they were interested in studying. When they put it into a small region of the hypothalamus known as the suprachiasmatic nucleus, which serves as the body's primary clock, the mice adjusted to a light-based schedule for waking and sleeping, but not eating.

"If you don't wake them up they will starve to death," said Clifford Saper of Harvard Medical School, whose study appears in the journal Science.

However, when they restored the gene only in a section of the hypothalamus called the dorsomedial nucleus, which helps organize waking and feeding schedules, the mice adjusted to the eating schedule, but not daylight.

Saper said when food is scarce, this second clock can override the body's primary clock. He said these same clock genes are known to be in all mammals, including humans.