The Impact of School Start Times on Adolescent Health and Academic Performance

A study published in the September 2010 issue of the journal “Sleep” found that teenagers who slept less than eight hours per weeknight ate more fatty foods and snacks than adolescents who slept eight hours or more. ⁽⁴⁷⁰⁾ For each one-hour increase in sleep duration, the odds of consuming a high amount of calories from snacks decreased by an average of 21 percent. ^{(129, 470)}

A significantly greater proportion of teens who slept less than eight hours per weeknight consumed food in the early morning between 5 a.m. and 7 a.m. ^{(129, 470)} According to Harvard Professor of Sleep Medicine Susan Redline, “Altered timing of eating in shorter sleepers also may be a metabolic stress that contributes to metabolic dysfunction.” ^{(129, 470)} Obviously, being awake for longer hours means more opportunities to eat. Weight gain may be promoted by eating at a time when circadian rhythm dictates sleep. ⁽⁴⁷¹⁾

A 2010 CDC study published in the Journal of the American Medical Association reports that the rate of obesity in U.S. adolescents between the ages of 12 and 19 years was 18.1 percent in 2007-2008. ⁽⁴⁷²⁾ The authors noted that the prevalence of high body mass index in childhood has remained steady for 10 years and has not declined despite coordinated prevention efforts. ⁽⁴⁷²⁾

Overweight children and adolescents tend to have reduced REM sleep. ⁽¹²⁸⁾ Although a recent study suggests otherwise, ⁽⁴⁷³⁾ Dr. Redline and other researchers surmise sleep loss may be the missing link ⁽¹²⁹⁾ in understanding why diet and exercise obesity interventions fail. ^{(127, 130, 474, 475)} 

Harvard Professor of Nutrition and Epidemiology Frank Hu observes that there is a close relationship between dietary habits and sleep habits. ⁽⁴⁷⁶⁾ Sleep is as important as diet and exercise. ⁽⁴⁷⁶⁾ Sleep-deprivation serves to increase production of the appetite-stimulating hormone, ghrelin, sending a signal to the brain to eat, particularly high calorie, high sugar-content foods. ⁽⁴⁷⁶⁾ By contrast, restricted sleep diminishes the production of leptin, a hormone which suppresses appetite. ⁽⁴⁷⁶⁾ Production of a third hormone, cortisone, is increased by sleep loss, elevating heart rate and blood pressure. ⁽⁴⁷⁶⁾ Chronic elevation of cortisone levels increases the risk of weight gain and obesity, with the latter being particularly dangerous for type 2 diabetes and cardiovascular disease. ⁽⁴⁷⁶⁾

A large-scale epidemiological study published in May 2012 found that beyond sleep duration, social jetlag is associated with increased BMI. ⁽⁴⁷⁷⁾ “Social jetlag quantifies the discrepancy that often arises between circadian and social clocks, which results in chronic sleep loss.” ^(477, citations omitted.) Led by Professor Till Roenneberg, Vice-Chair and Head of Human Chronobiology at the Institute of Medical Psychology in Germany, the study demonstrates:

“[L]iving ‘against the clock’ may be a factor contributing to the epidemic of obesity. This is of key importance in pending discussions on the implementation of Daylight Saving Time and on work or school times, which all contribute to the amount of social jetlag accrued by an individual. Our data suggest that improving the correspondence between biological and social clocks will contribute to the management of obesity.” ⁽⁴⁷⁷⁾