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Epidemiologic studies suggest that people who habitually sleep fewer than 6 hours nightly, or whose sleep patterns do not correspond to normal circadian rhythms, have excess risk for developing obesity and type 2 diabetes. Researchers conducted a controlled study to confirm and explain this observation. Twenty-one participants lived in individual quarters in a sleep laboratory for 6 weeks. They had no contact with the outside world (including cues about nighttime and daytime); timing of meals, caloric intake, and activity levels were experimentally controlled. For the first 3 weeks, 10 hours of darkness (“sleep opportunity”) occurred during what was nighttime in the outside world. For the next 3 weeks, the amount and timing of darkness was …