Most people become aware of their internal circadian clock when they cross several time zones and experience jet lag, but scientists have known for decades that the rhythm of the internal human clock regulates almost every biological system, from blood pressure to sex drive.

Now researchers at Stanford University have shown that when the circadian system breaks down, so does memory. In a paper published in the Proceedings of the National Academy of Sciences, biologist Norman Ruby found that hamsters with disabled circadian systems were, unlike "normal" hamsters, unable to remember their environment.

"They can't remember anything," Ruby said of the hamsters.

Scientists have long suspected that learning and memory might be related to different levels of brain function, or alertness, that change over a day due to normal circadian rhythms, but it hadn't been shown that the circadian system is crucial to learning and memory. Ruby found that learning and memory appears to hinge on the amount of the neurochemical GABA, which is found in the brains of all animals. GABA, which inhibits brain activity, is released rhythmically by the body in accordance to the circadian clock controlling sleep and wake cycles.

When Ruby disabled the hamsters' clocks by manipulating their exposure to light the hamsters experienced chronically high levels of GABA and essentially lost their ability to remember. The findings have implications for people with Down syndrome, who grow up with what amounts to an over-inhibited brain, according to the research. It also may have implications for the decline in memory that older adults often experience.

"In aging humans, one of the big things that happens is the circadian system starts to degrade and break down," Ruby said. "It might be that the degradation of circadian rhythms in elderly people may contribute to their short-term memory problems."

Inside Science News Service is supported by the American Institute of Physics.