Each subject selected for laboratory evaluation completed a comprehensive sleep history and physical examination. All subjects were evaluated for one night in the sleep laboratory in sound-attenuated, light- and temperature-controlled rooms. During this evaluation, each subject was continuously monitored for 8 h (fixed-time period) using 16-channel polysomnography including electroencephalogram, electrooculogram, and electromyogram. Bedtimes were adjusted to conform to subjects' usual bedtimes, and subjects were recorded between 22:00-23:00 and 06:00-07:00. The sleep recordings were subsequently scored independently, according to Rechtschaffen and Kales criteria.30 Percent of sleep time is total sleep time divided by recorded time in bed and multiplied by 100. From the objectively recorded sleep time data, we regrouped the entire study sample into 2 ordinal groups: the top 50% of persons above the median percent of sleep time (“longer sleep duration group”), and the 50% of persons in the bottom half (“short sleep duration group”). We then rounded the cutoff point to meaningful numbers and thus created the following 2 sleep duration groups: the “longer sleep duration group” consisted of those who slept ≥ 6 h (i.e., percent of sleep time ≥ 75%), and the “short sleep duration group” of those who slept < 6 h (i.e., percent of sleep time < 75%). This cutoff point has been shown in previous studies to be predictive of significant medical morbidity and mortality.20,21–23
Additional information obtained during the PSG included that assessing sleep apnea. Respiration was monitored throughout the night by use of thermocouples at the nose and mouth and thoracic strain gauges. All-night recordings of hemoglobin oxygen saturation (SpO2) were obtained with an oximeter attached to the finger. For the purpose of this study, sleep disordered breathing (SDB) was defined as an apnea or hypopnea index ≥ 5 (AHI ≥ 5). Body mass index was based on measured height (cm) and weight (kg) during the subjects' sleep laboratory visit, and data are presented in terms of mean and percentage within each category.