Many of our behaviors, like sleeping or eating, follow a roughly daily routine or circadian rhythm. Internal factors--oscillating gene products in the body--set these rhythms, and external signals--for example, the change in light from day to night--synchronize and shape them. Scientists from the U.S., Norway and Japan have now discovered that restricted feeding of rats can reset the circadian clock in their liver without disturbing it in other parts of their body.

The researchers measured the oscillating expression of Per1, a "clock gene," in three organs: in the suprachiasmatic nucleus (SCN), a part of the hypothalamus that receives day/night signals and is widely regarded as the "master clock"; in the lungs; and in the liver. Then they started offering the rats, who could previously eat whenever they wanted, food only during a set four hours every day, measuring the effect on both their behavior and on Per1 expression. In last week's issue of Science, they report that the rats became more active just before they received food and also at night. But only the circadian clock in the liver--and to a smaller extent in the lungs--were shifted. An extended eight-hour feeding period affected only the liver clock. A stress hormone had no effect on either liver or lung Per1 expression, suggesting that the main reason for the shift was not the stress associated with restricted mealtimes but rather feeding itself.

Based on these results, it appears as if the liver of mammals responds directly, and independently from the "master clock" in the SCN, to changes in the environment--in this case to feeding patterns. This could be useful for travelers or shift workers, who might be able to adjust this part of their body clock simply by controlling their mealtimes.

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