Length polymorphism in the Period 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 34/4 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