SEATTLE, June 12 (UPI) -- Study participants who slept between 7 and 8.9 hours nightly had a lower mean body mass index than those who slept fewer than 7 hours, U.S. researchers said.
The researchers found those who slept more than 8.9 hours also had a higher mean body mass index.
Lead author Dr. Nathaniel Watson of the University of Washington Sleep Institute in Seattle said sleep habits have a significant impact on weight and body mass index -- a measure physicians use to determine body fat that is calculated using height and weight.
"Findings of the study point towards an environmental cause of the relationship between sleep duration and BMI," Watson said in a statement. "Results were robust enough to be present when the sample was limited to identical twins."
The study included data from 1,797 twins, including 634 twin pairs -- 437 monozygotic, when a single fertilized egg splits into two embryos; 150 dizygotic, or when two separate eggs are fertilized by separate sperm and 47 indeterminate pairs and 529 individual twins with a mean age of 36.8.
The study included data from 1,797 twins, including 634 twin pairs and 529 individual twins with a mean age of 36.8. Habitual sleep duration was self-reported, as was height and weight. Results persisted in a co-twin control analysis of within twin pair differences in sleep duration and body mass index.
The findings were presented at Sleep, the 23rd annual meeting of the Associated Professional Sleep Societies in Seattle.