Chronotype in relation to shift work: A cohort study among 37,731 female nurses.

Chronotype may affect tolerance for circadian disruption induced by shift work. This study examines the association between chronotype, self-reported sleep timing, shift type preference, and sleep problems among nurses, and studies chronotype stability over time. The study included 37,731 Dutch female nurses who completed a baseline (2011) and follow-up questionnaire (2017), with information on shift work (e.g., job history, shift type preference [collected in 2017 only]), and sleep characteristics (e.g., chronotype, preferred sleep-wake time in a work-free period [collected in 2017 only], and sleep problems between working days according to Medical Outcomes Study-Sleep Problem Index II [MOS-SPI-II]). The association between chronotype and sleep timing was examined using (age-adjusted) linear regression. Associations between chronotype and shift type preference and sleep problems (MOS-SPI-II >30) were examined using ordered logistic and Poisson regression, respectively. With later chronotype, midsleep time increased (definite evening vs. intermediate types [reference]: ß = 55 min, 95% confidence interval [95% CI]: 54-55), the odds ratio (OR) for 1-point increase in preference for night (2.68; 95% CI: 2.48-2.90) and evening shifts increased (OR 2.20; 95% CI: 2.03-2.38), while the odds for day (OR 0.17; 95% CI: 0.16-0.18) and morning shifts (OR 0.22; 95% CI: 0.21-0.24) decreased. Intermediate chronotype was associated with fewer sleep problems (median MOS-SPI-II = 27.2, p < 0.01), compared with definite morning (28.9) and evening types (31.7). This study shows that chronotype is associated with sleep-wake times in a work-free period, shift type preference, and sleep problems in nurses. Future studies on the association of shift work-induced circadian disruption and health outcomes should therefore consider chronotype as effect-modifier.

Single-item chronotype is associated with dim light melatonin onset in lymphoma survivors with fatigue.

Chronotype is frequently assessed in human observational studies using various morningness-eveningness questionnaires. An alternative single-item chronotype question has been proposed for its reduced administration time and its accessibility to all types of populations. We investigated whether this single-item chronotype is associated with dim light melatonin onset, the "gold standard" for estimating the endogenous circadian phase. We used data from a randomised trial in 166 (non-)Hodgkin lymphoma survivors with cancer-related fatigue. All participants completed a questionnaire, including a single-item chronotype question. A subsample of 47 participants also provided saliva samples before sleep onset for melatonin measurement. Using multiple linear regression, we examined whether chronotype based on a single question was associated with dim light melatonin onset. The subsample of 47 participants had a mean age of 44.6 years. The mean (SD) dim light melatonin onset was at 8:42 (1:19) p.m. and the most common chronotype was more evening than morning person (29.2%). A gradual increase in dim light melatonin onset with later chronotype (i.e. evening preference) was observed, with a mean ranging from 7:45 p.m. in definite morning persons to 9:16 p.m. in definite evening persons. Our study shows that single-item chronotype is associated with dim light melatonin onset as a marker of the endogenous circadian phase of fatigued lymphoma survivors. This type of chronotype assessment can therefore be a useful alternative for more extensive morningness-eveningness questionnaires.