Inside a small chamber at a Kent State University laboratory, hamsters sleep, eat, play and rest while fluid flows in and out of tubes threaded through their tiny brains. It took biology professor J. Da vid Glass two years to set up the finicky dialysis system, which measures a key neurotransmitter in the biological clocks of these nocturnal rodents. His payoff came in 1996, when he became the first re searcher to measure serotonin levels rising and falling in the biological clock area of the brain during an animal's daily cycle. Serotonin is the 'feel good' chemical manipulated by widely prescribed drugs such as Prozac. Meanwhile, in a larger chamber down the hall, Glass is monitoring tropical monkeys. He has found that exercise and arousal from sleep have major impacts on the biological rhythms of the monkeys, permanently shifting their clocks in the absence of normal daylight and darkness cues. Glass's research and that of others could have implications for the millions of people who take com monanti-depressants and other drugs that affect serotonin in the brain. It has long been known that the substance is a key player in the biological clock, and that the region has an unusually high concentration of receptors for the neurotransmitter. Glass's work is part of the fast-growing field of circadian(or daily)rhythm research focused on a region at the base of the brain, the size of a corn kernel, that scientists discovered 25 years ago is the body's timing mechanism. Like other animals and even plants, humans have built-in clocks that regulate internal functions on a 24-hour basis. For most mammals, the clocks trigger sleep and waking, as well as metabolism, hor mone levels, body temperature and many other changes. This is a particularly exciting time for circadian-rhythm researchers. In recent times, scientists at universities in Illinois, Texas and Japan have found genes involved with the clock, including one that appears to be a basic building block of the mechanism and is common across all species, from fruit flies to humans. Meanwhile, researchers like Glass, whose work has attracted US $1.2 million(9.6 mil lion RMB)in grants from the National Institute of Health, are trying to understand how the clock works. Sitting on top of the optic nerve, the clock is heavily influenced by light. But other factors, too, are involved in resetting the mechanism, most notably physical activity and substances like serotonin. Glass and his students found that, when lights in the hamster chamber were switched off, the serotonin levels in the rodents'clock region shot up: hamsters are nocturnal, meaning they rest during the day and are awake at night. But when hamsters in the midst of their sleep cycle were put onto an activity wheel, a significant rise in serotonin levels was measured in those hamsters that woke up enough to ex ercise. It has long been known that serotonin is key to body clock function, according to Thomas Wehr, a scientist at the National Institute of Mental Health in Bethesda, Maryland. Researchers at the Mary land Institute took cells from the clock region of the brain, sprinkled serotonin on them and, by monito ring electrical impulses, watched the cells 'reset' themselves. 'There are certain drugs used with humans that have also been squirted on these cells in dishes and have been shown to reset the clock in the dish, so it seems quite possible there are similar effects in hu mans who take these drugs. 'Wehr says. Indeed, some people taking anti-depressants do report sleep disorders such as insomnia or daytime drowsiness that could be related to changes in their biological clocks. Human studies have yet to focus on the issue. Studies have found that serotonin affects the clock in different ways, depending on the point in the cell's daily cycle that it is administered. Glass rece