Familial sleep traits

Familial sleep traits are heritable variations in sleep patterns, resulting in abnormal sleep-wake times and/or abnormal sleep length. Circadian rhythms are coordinated physiological and biological changes that oscillate on an approximately 24-hour cycle. Disruptions to these rhythms in humans may affect the duration, onset, and/or quality of sleep during this cycle, resulting in familial sleep traits. These traits are not necessarily syndromes because they do not always cause distress among individuals. Instead of being disorders, familial sleep traits are variations in an individual's biological tendencies of sleep-wake times, and are only considered syndromes if affected individuals complain about life interference, in which case they may fall under the category of Circadian Rhythm Sleep Disorders (CRSD) that affect sleep timing and circadian rhythms. Some of these circadian disorders include Advanced Sleep Phase Disorder (ASPD) and Delayed Sleep Phase Disorder (DSPD). Familial sleep traits are more specific than CRSD because they are heritable and involve a wide range of Mendelian genes. Evidence has shown that genes significantly influence sleep schedules in mammals, including humans, and account for one-third of the variation in sleep quality and duration. Studies in human monozygotic twins have provided evidence that genetic factors affect "normal" sleep patterns as well, meaning ones where no individual has been diagnosed with an altered phenotypic sleep trait. Sleep timing is controlled by the circadian clock, which can entrain to environmental stimuli (usually a light-dark cycle) and is regulated by a transcription-translation feedback loop (TTFL). In humans, there are multiple genes involved in this molecular biological clock, which when mutated may result in sleep disorders such as Familial Advanced Sleep Phase (FASP), Familial Delayed Sleep Phase (FDSP), and Familial Natural Short Sleep (FNSS). Some mutations in Mendelian genes that are involved in the TTFL have been identified as the causes of these sleep traits, including PER2, PER3, CRY2, CRY1.