Although sleep diaries are widely used in clinical and research settings, only a few studies have compared the subjective information recorded in these diaries to objective information about sleep recorded. The goal of this study was to determine if a sleep diary could be used to obtain reliable data about home sleep/wake patterns over a 24-hour period. Fifty subjects (25 narcoleptic and 25 matched control subjects) completed a sleep diary while undergoing 24-hour ambulatory polysomnographic monitoring. The percentage agreement between the subjective data recorded in the sleep diaries and polysomnographic data was acceptable (kappa = .87). Sensitivity and specificity were also high (92.3% and 95.6%). The sleep diary is a reliable instrument for collecting data about sleep/wake patterns, but should be used with caution when collecting data from subjects who are likely to take frequent daytime naps.