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The bodys internal clock helps to regulate a water-storing hormone so that nightly dehydration or trips to the toilet are not the norm, research suggests. In an article published in Nature Neuroscience today, neurophysiologists Eric Trudel and Charles Bourque at the Research Institute of the McGill University Health Centre in Montreal, Canada, propose a mechanism by which the bodys circadian system, or internal clock, controls water regulation1. By allowing cells that sense water levels to activate cells that release vasopressin, a hormone that instructs the body to store water, the circadian system keeps the body hydrated during sleep. Weve known for years that theres a rhythm of vasopressin that gets high when youre sleeping. But no one knew how that occurred. And this group identified a very concrete physiological mechanism of how it occurs, says Christopher Colwell, a neuroscientist who studies sleep and circadian rhythms at the David Geffen School of Medicine at the University of California, Los Angeles. The body regulates its water content mainly by balancing water intake through thirst with water loss through urine production. People dont drink during sleep, so the body has to minimize water loss to remain sufficiently hydrated. Scientists knew that low water levels excite a group of cells called osmosensory neurons, which direct another set of neurons to release vasopressin into the bloodstream. Vasopressin levels increase during sleep; clock neurons, meanwhile, get quieter. | 
