WE ARE not slaves to light after all. People have a second body clock, dictated by our need for food, which can overcome the light-activated circadian clock that鈥檚 supposed to control when we rise and sleep.
That, at least, is the opinion of Steven McKnight from the University of Texas Southwestern Medical Center in Dallas. He has found a clock system in the brain which he believes responds to how much food and energy is available, rather than night and day.
The clock is a protein, called NPAS2, which is mainly found in the forebrain. It controls the production of enzymes that determine whether our cells build up fuel stores or burn them, he says. Although the clock may have a natural daily rhythm, it is 鈥渆ntrained鈥� or reset according to the availability of energy.
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Since the protein is in the forebrain, it could affect the activity of neurons, firing them up during the day and slowing them down at night. But it appears the food clock鈥檚 main function is to allow us to break ranks with the light signal and alter our behaviour and activity in response to energy needs. 鈥淚f you lose the forebrain clock, you become a slave to light,鈥� he says.
His conclusions come from experiments with mice. The mice only got food during the day, when they are usually asleep. Normal mice soon shifted their body clock to feed and run during the day. But mice without NPAS2 stuck to their nocturnal habits so persistently that they simply starved to death if no one intervened.
But while other researchers accept that food can alter body rhythms, not everyone believes there鈥檚 a specific high-level clock that responds to energy needs. It鈥檚 really intriguing that a lack of NPAS2 can alter the response to restricted feeding times, says Amita Sehgal of the University of Pennsylvania Medical School in Philadephia. But she鈥檚 not convinced that the food clock is on a par with the light-entrained body clock. Light is the most powerful signal, she believes. And the light clock is extremely robust.
McKnight disagrees. 鈥淔ood is more important than light.鈥� To avoid jet lag, for instance, you need to eat at the right time for your new time zone.
