Depression as a confounding variable in the estimation of habitual sleep time.

Self-reported habitual sleep time is an important variable because short and long sleep times are associated with mortality. Speculation with regard to these results usually focuses on physical health, rather than psychological factors. We investigated the role of anxiety and depression in reports of habitual sleep times by examining the relative and absolute discrepancy between individuals' initial estimates of their sleep times and sleep diaries made over a 2-week period. Results indicated that depressed mood was associated not only with a tendency initially to underestimate length of sleep (relative discrepancy), but also to exaggerate reported sleep time regardless of direction (absolute discrepancy). These results imply that studies that examine relationships between reported sleep times and mortality should take mental health factors into account.