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Circadian Biology in Humans | Circadian Rhythms and Sleep Disorders

Polymorphisms in clock genes can be related to circadian parameters.  The advanced sleep phase syndrome (ASPS) can be inherited as a single-gene point mutation.  This has been verified in a large family with this circadian periodicity of early awakening and early bedtime.  Affected family members demonstrated advanced melatonin release, advanced temperature rhythms and advanced sleep/wake rhythms due to a mis-sense mutation in the Per2 gene.   This resulting PER2 protein is phosphorylated less efficiently than the “common” type.  This mutation has also been discovered in the same locus in hamsters, resulting in the same circadian abnormality.

Some people are naturally more alert in the morning, whereas others are more alert in the evening.   Unlike individuals with morning preference such as those discussed above with ASPS, the gene associated with evening preference that has produced the most interesting results to date in humans is Per3.  A single-point nucleotide polymorphism causing an amino acid substitution has been found to correlate with delayed sleep phase syndrome in a Japanese and a British population.

It appears, therefore, that there may not always be a true choice in “morning preference” versus “night preference”, and it is unfortunate that society has associated early awakening with merit and diligence, whereas late sleepers are thought to be slothful and idle.  Based on these recent findings of the genetic and molecular control of circadian rhythms, some people are merely genetically programmed for a certain pattern of sleep as a result of their mechanisms of circadian control.