Non-24-Hour Sleep-Wake Disorder (N24HSWD) occurs when individuals, primarily those without light perception, are unable to synchronize their endogenous circadian pacemaker to the 24-hour light-dark cycle, and the timing of their circadian rhythm instead reflects the intrinsic period of their endogenous circadian pacemaker. As a result, the circadian rhythm of sleep-wake propensity in these individuals moves gradually later and later each day if there circadian period is > 24 hours and earlier and earlier if < 24 hours. These individuals will be able to sleep well at night when their sleep-wake propensity rhythm is approximately aligned with the 24-hour light-dark and social cycle. However, after a short time, the endogenous sleep-wake propensity rhythm and the 24-hour light-dark cycle will move out of synchrony with each other, and they may have difficulty falling asleep until well into the night. In addition to problems sleeping at the desired time, the subjects experience daytime sleepiness and daytime napping.