We excluded 22 of 669 participants from the main analysis for these reasons: 19 lacked either weekday or weekend recordings, 1 did not complete the sleep questionnaires, 1 appeared to have removed the actigraph during the night and one outlier for whom almost no sleep was recorded. Thus the final sample for wave 1 analysis comprised 647 subjects (). Mean measured sleep duration was 6.06 hours. Mean self-reported habitual sleep was 6.83 hours, and only 17 percent reported less than the measured mean.
Study sample characteristics for wave 1 of the sleep study, Chicago site of CARDIA, 2003-2004.
For habitual sleep in wave 1, the bias at the mean of 6 hours measured sleep was 0.80 hours (48 minutes), with subjective reports longer than measured sleep (). The calibration, represented by the beta coefficient, was substantially less than one: for each additional hour of mean sleep recorded, report of habitual sleep increased, on average, by 31 minutes. Mean measured sleep explained 20 percent of the variation (r2 = 0.20) in reported habitual sleep, a correlation of 0.45. Combining the effects of bias and calibration, persons who slept 5 hours reported, on average, 6.29 hours of sleep, and persons who slept 7 hours reported 7.31 hours.
Table 2 Linear regression models predicting reported habitual sleep hours by average sleep duration measured using wrist actigraphy. Regression is a modified version of errors-in-variables regression. Data collected in 2000-2004 from the Chicago site of CARDIA. (more ...)
Bias varied little by sex, education, income or sleep variability, but did vary significantly by several demographic, health and sleep variables (). The bias was closer to 0 for blacks, the obese, those with high depression scores, high apnea risk, high sleepiness, and high sleep efficiency. The stratification that made the greatest difference in bias was apnea risk, with low risk persons overestimating sleep by an average of 54 minutes, and high risk persons overestimating sleep by only 10 minutes.
Calibration did not vary significantly for most of the stratification variables, but was better (closer to one) for those with higher sleep efficiency (). An additional hour of measured sleep corresponded to 47 minutes more of reported sleep for those with higher sleep efficiency, but 25 minutes more of reported sleep for those with low sleep efficiency. Calibration was also better (although the comparisons were not statistically significant) for those with higher incomes, more education and older age. Similarly, the r2 was highest (.29 to .34, corresponding to correlations between .54 and .58) for whites, persons older than the mean, those with more education, more income, and higher sleep efficiency. The correlation was lower (between .20 and .40) for blacks, persons younger than the mean, without a college degree, with low income or low sleep efficiency. Sleep variability did not significantly affect bias, calibration or discrimination. Both calibration and correlation were close to 0 for persons in fair/poor health. Since less than 10% of the sample was in this category, only the correlation contrast attained statistical significance.
Because of lower participation in wave 2, the final sample of single-night sleep was 615 subjects. For a single night, the bias was 0.63 hours (38 minutes), with subjective reports longer than measured sleep (data not shown). For each additional hour of sleep recorded, the report of sleep duration increased by 35 minutes. Measured sleep explained 36 percent of the variation in reported sleep for a single night, a correlation of 0.60.
The sensitivity analysis identified 216 actigraphic records where sleep for at least one night would have been 30 minutes longer if the period before the recorded bedtime and after the recorded wake time were scanned for inactivity that resembled sleep. However, repeating the main analysis of habitual objective-subjective sleep without these records did not improve the estimates of bias, calibration or discrimination. The bias of the remaining records was 0.86 hours, the calibration was .45 and the discrimination was .17 (data not shown). Stratified results were also very similar to the full sample, although some comparisons were no longer statistically significant, consistent with smaller sample sizes.