Length polymorphism in the P eriod 3 gene is associated with sleepiness and maladaptive circadian phase in night‐shift workers

Summary The objective of the current study was to determine if night‐shift workers carrying the five‐repeat variant of the Period 3 gene show elevated levels of nocturnal sleepiness and earlier circadian phase compared with homozygotes for the four‐repeat allele. Twenty‐four permanent night‐shift workers were randomly selected from a larger study. Participants took part in an observational laboratory protocol including an overnight multiple sleep latency test and half‐hourly saliva collection for calculation of dim‐light melatonin onset. Period 3 –/5 shift workers had significantly lower multiple sleep latency test during overnight work hours compared with Period 3 4/4 workers (3.52 ± 23.44 min versus 10.39 ± 6.41 min, P  =   0.003). We observed no significant difference in sleepiness during early morning hours following acute sleep deprivation. Long‐allele carriers indicated significantly higher sleepiness on the Epworth Sleepiness Scale administered at 17:00 hours (12.08 ± 2.55 versus 8.00 ± 1.94, P  <   0.001). We observed a significantly earlier melatonin onset in Period 3 –/5 individuals compared with Period 3 4/4 shift workers (20:44 ± 6:37 versus 02:46 ± 4:58, P  =   0.021). Regression analysis suggests that Period 3 genotype independently predicts sleepiness even after controlling for variations in circadian phase, but we were unable to link Period 3 to circadian phase when controlling for sleepiness. Period 3 –/5 shift workers showed both subjective and objective sleepiness in the pathological range, while their Period 3 4/4 counterparts showed sleepiness within normal limits. Period 3 –/5 night workers also show a mean circadian phase 6 h earlier (i.e. less adapted) than Period 3 4/4 workers. Because Period 3 –/5 workers have maladaptive circadian phase as well as pathological levels of sleepiness, they may be at greater risk for occupational and automotive accidents. We interpret these findings as a call for future research on the role of Period 3 in sleepiness and circadian phase, especially as they relate to night work.