A University of Pennsylvania study of 225 people who spent 18 days in a sleep lab showed that those who slept four hours, from 4 a.m. to 8 a.m., for five nights gained more weight than did control subjects who slept from 10 p.m. to 8 a.m. The study, published in the journal Sleep, found that people in the restricted-sleep group consumed 550 more calories from 10 p.m. to 4 a.m. compared with regular sleepers, and that the proportion of calories attributed to fat was higher than at other times during the day.

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