PERIOD3, circadian phenotypes, and sleep homeostasis

SummaryCircadian rhythmicity and sleep homeostasis contribute to sleep phenotypes and sleep–wake disorders, some of the genetic determinants of which are emerging. Approximately 10% of the population are homozygous for the 5-repeat allele (PER35/5) of a variable number tandem repeat polymorphism in the clock gene PERIOD3 (PER3). We review recent data on the effects of this polymorphism on sleep–wake regulation. PER35/5 are more likely to show morning preference, whereas homozygosity for the four-repeat allele (PER34/4) associates with evening preferences. The association between sleep timing and the circadian rhythms of melatonin and PER3 RNA in leukocytes is stronger in PER35/5 than in PER34/4. EEG alpha activity in REM sleep, theta/alpha activity during wakefulness and slow wave activity in NREM sleep are elevated in PER35/5. PER35/5 show a greater cognitive decline, and a greater reduction in fMRI-assessed brain responses to an executive task, in response to total sleep deprivation. These effects are most pronounced during the late circadian night/early morning hours, i.e., approximately 0–4 h after the crest of the melatonin rhythm. We interpret the effects of the PER3 polymorphism within the context of a conceptual model in which higher homeostatic sleep pressure in PER35/5 through feedback onto the circadian pacemaker modulates the amplitude of diurnal variation in performance. These findings highlight the interrelatedness of circadian rhythmicity and sleep homeostasis.