Sleep Timing and Insulin Resistance in Adolescents With Obesity

Primary outcomes

1. Dim Light Melatonin Onset and Offset

   Time frame: 1 day

   ~1mL saliva was collected at 30- to 60- minute intervals in dim light (<5 lux in the angle of gaze, approximately the light level of candlelight or civil twilight) from approximately 5pm until noon the next day. Dim light melatonin onset (DLMOn) was defined as the linear interpolated clock time at which evening salivary melatonin concentrations increased and remained above a threshold of 3pg/mL. Melatonin offset (DLMOff) was the linear interpolated clock time at which salivary melatonin concentrations fell below this threshold. Later DLMOn and DLMOff are indicative of a later circadian rhythm.

2. Insulin Sensitivity

   Time frame: 3 hours

   After an overnight fast, participants completed an oral glucose tolerance test (OGTT) in the morning. Participants consumed a 75g dextrose drink and serum for glucose and insulin concentrations were collected at baseline and every 30 minutes for 3 hours. The homeostatic model assessment for insulin resistance (HOMA-IR) was calculated as [fasting insulin (μU/ml) x fasting glucose (mmol/l)] / 22.5); lower HOMA-IR indicates better insulin sensitivity. The Matsuda Index was calculated as √10,000 / [[fasting insulin (μU/ml) x fasting glucose (mmol/l)] x [mean OSTT insulin (μU/ml) x mean OSTT glucose (mmol/l)]]; high Matsuda Index indicates better insulin sensitivity.