Familial advanced sleep phase disorder (FASPS) is characterised by the presence of a recurrent pattern of early night-time sleepiness and early awakening. This results in an earlier sleep phase by 4-6 hours compared to the average. In many cases, this result in a sleep deficit leading to excessive daytime sleepiness. Patients with FASPS experience normal quality and quantity of sleep, however, sleep deprivation may be imposed by social norms that cause individuals to delay bedtime. This can therefore significantly interfere with the affected person's social and work life, leading to emotional and physical disturbances.
Although sleep-wake cycle disturbances in adults are relatively common, the extreme phase advance typical of this syndrome is rare and is estimated to occur in 1% of middle-aged adults, regardless of gender.
Although sleep phase advancement can occur for various reasons and can be sporadic, there is a familial form described that is hereditary and follows an autosomal dominant pattern of inheritance, so that a history of affected individuals within the same family is very common in these cases. To date, some causative mutations have been identified in the PER2 and CSNK1D genes, both of which are involved in the regulation of circadian rhythm.