Familial sleep traits are more specific than CRSD because they are heritable and involve a wide range of Mendelian genes.
[1] 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).
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.
[4][5] With some familial sleep traits, there may be a shift in an individual's chronotype, which describes the time of sleep-wake behaviors that result from circadian rhythms.
Researchers have examined the human prevalence of FASP to be 0.33-0.5% by including individuals who have a sleep onset at approximately 8:30pm and offset at 5:30am.
FDSP, which includes individuals who have a delayed sleep onset and offset, has an unknown human prevalence and may vary based on location, definition, and age.
[6] Familial sleep traits have been difficult to study due to the various environmental influences (such as entraining daily alarms, artificial light at night, and caffeine or stimulant intake) that can contribute to different behavioral phenotypes in humans.
Despite these potential difficulties, Louis Ptáček and colleagues discovered evidence of a human familial circadian rhythm variant in the 1990s.
Since this initial 1999 FASP publication, other circadian biologists including Phyllis Zee and Joseph Takahashi have conducted further genetic analysis.
They published a paper in 2001 that presented data showing a phenotypically characterized case of Advanced Sleep Phase Syndrome to provide further evidence that this trait can be hereditary.
[10][11] Almost two decades later in 2017, Michael Young and colleagues in New York published findings that further supported delayed sleep to have a genetic component, resulting in FDSP.
These scientists reported that a mutation in CRY1, a component of the TTFL that represses Clock and Bmal1, results in a gain-of-function variation that lengthens circadian period.
In 2019, Ptáček and Fu published further research about the genetic aspect of FNSS, identifying a mutation in the gene ADRB1 that increases the activity of ADRB1+ neurons in the dorsal pons.
[4][13] Most of the research conducted thus far has been surrounding FASP, FDSP, and FNSS, with recent studies beginning to examine the roles of heritable sleep variability on autism-spectrum disorder (ASD) and Alzheimer's disease (AD).
[2] ASD, a neurodevelopmental disorder, has evidence of genetic components and affected individuals have reported a high prevalence of insomnia.
Fu, Ptáček, and colleagues have hypothesized that it may be interesting to examine if sleep traits and disruptions can exacerbate the atypical neurodevelopment in ASD.
[2] Both ASD and AD demonstrate how the heritability of sleep traits may also be involved in disorders and diseases that are not traditionally thought of as circadian, but more research must be done in this field.
[2] The functions of heritability for many sleep traits are not well known, underscoring the importance of continued research into the human genome.
This also means that certain physiological markers, such as body temperature and melatonin will be present at higher levels earlier in the day as compared to an average person.
Additionally, individuals may experience excessive daytime sleepiness if they are forced to adhere to a schedule offset from their personal biological clock.
Individuals with FASP typically need to have a two-hour delay per day to remain entrained, due to their 22-hour period.
[3] Fatal Familial Insomnia (FFI) is a disorder that results in trouble sleeping, speech and coordination problems, and eventually dementia.
Additionally, affected individuals have a higher behavioral drive, resulting in many holding high pressure jobs, and they may have a better ability to deal with stress.
[24] In 2009, Ying-Hui Fu and colleagues described how a genetic variant in DEC2 produced the short sleep phenotype.
[23] Understanding how these individuals are able to tolerate higher sleep pressure and behavioral drive will prove useful for numerous people that hold jobs which require long durations of wakefulness.