Resetting the human biological clock -- circadian rhythm -- is a lot harder than resetting a wristwatch or clocks, a U.S. researchers said. Researchers at the National Institute of General Medical Sciences, part of the National Institute of Health, said circadian rhythms determine sleep patterns and contribute to jet lag as well as any grogginess after a time change. Adjusting to daylight saving time might take longer when time advances in the spring, instead of when time rolls back in the fall, the researchers said. Biological clocks aren't made of cogs and wheels, but rather of groups of molecules in cells throughout the body. A "master clock" keeps everything in synch. In vertebrates, including people, the master clock is located in the brain -- the hypothalamus. Researchers at the University of California, Irvine, discovered that a protein -- named CLOCK -- is an essential component in directing circadian rhythms in humans, fruit flies, mice, fungi and other organisms. However, counterbalancing CLOCK is a metabolic protein called SI RT1 that senses energy use in cells. Upsetting the CLOCK-SI RT1 equilibrium can lead to sleep disruption and increased hunger and if the proteins remain chronically unbalanced, it can contribute to obesity, the researchers said.