Increasing night shifts from two to four resulted in greater sleep debt for early and lower sleep debt for late chronotypes.
Sleep debt varied based on chronotype, with early types accumulating more sleep loss over four consecutive night shifts.
Late chronotypes experienced reduced sleep debt after four night shifts, indicating a different response to shift length.
No significant interaction effects were observed between shift sequence length and chronotype for morning shifts.
Social jetlag remained unchanged regardless of the number of consecutive night shifts.
Including naps did not alter the relationship between shift length and sleep debt.
Simplified
The study aimed to explore -specific effects of two versus four consecutive morning or night shifts on sleep-wake behavior. Sleep debt and social jetlag (a behavioral proxy of circadian misalignment) were estimated from sleep diary data collected for 5 weeks in a within-subject field study of 30 rotating night shift workers (29.9 ± 7.3 years, 60% female). Mixed models were used to examine whether effects of shift sequence length on sleep are dependent on chronotype, testing the interaction between sequence length (two vs. four) and chronotype (determined from sleep diaries). Analyses of two versus four morning shifts showed no significant interaction effects with chronotype. In contrast, increasing the number of night shifts from two to four increased sleep debt in early chronotypes, but decreased sleep debt in late types, with no change in intermediate ones. In early types, the higher sleep debt was due to accumulated sleep loss over four night shifts. In late types, sleep duration did not increase over the course of four night shifts, so that adaptation is unlikely to explain the observed lower sleep debt. Late types instead had increased sleep debt after two night shifts, which was carried over from two preceding morning shifts in this schedule. Including naps did not change the findings. Social jetlag was unaffected by the number of consecutive night shifts. Our results suggest that consecutive night shifts should be limited in early types. For other chronotypes, working four night shifts might be a beneficial alternative to working two morning and two night shifts. Studies should record shift sequences in rotating schedules.
Key numbers
5.4 h
Increase in Sleep Debt for Early
Average cumulative sleep loss after two night shifts.
3.1 h
Decrease in Sleep Debt for Late
Average cumulative sleep loss after two night shifts.
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Conflict of Interest Statement: The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: D.F. and T.R. declare no conflicts of interest. C.V., during the part of this work, received research support from the NIH, the Colorado Clinical and Translational Sciences Institute, and the University of Colorado Boulder; was an unpaid scientific advisory board member of Circadian Light Therapy Inc. and Chronosulting; and served as a paid consultant to the U.S. Department of Energy and NIOSH outside of the submitted work. Study design, data processing and analysis, and result presentation and interpretation were independent from third parties.
