On the television show “24” a silent countdown usually means a character has died. But for some cells in the brain’s time-keeping center, silent running is part of life.

Cells in the suprachiasmatic nuclei, a tiny group of neurons in the hypothalamus, serve as a master clock to regulate the body’s circadian rhythms — daily fluctuations in hormone release, body temperature, blood pressure and other processes — and help set meal and bed times. The cells follow a predictable daily pattern, firing electrical signals faster during the day and quieting at night. Or so scientists thought.

A new study shows that some cells in the SCN work themselves into a frenzy and then fall silent in the middle of the afternoon, a pattern most scientists did not expect and one that would kill most brain cells. The study appears in the Oct. 9 Science and shows that the SCN contains at least two different populations of neurons, each having its own rhythm and one showing this extreme behavior.

The finding is “actually very shocking,” says Martha Gillette, a neuroscientist at the University of Illinois at Urbana-Champaign who was not involved with the study.

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