The impact of chronotype on circadian rest-activity rhythm and sleep characteristics across the week

ABSTRACT

Circadian rhythms are maintained by a complex “system of systems” that continuously coordinates biological processes with each other and the environment. Although humans predominantly entrain to solar time, individual persons vary in their precise behavioral timing due to endogenous and exogenous factors. Endogenous differences in the timing of individual circadian rhythms relative to a common environmental cue are known as chronotypes, ranging from earlier than average (Morningness) to later than average (Eveningness). Furthermore, individual behavior is often constrained by social constructs such as the 7-day week, and the “sociogenic” impact our social calendar has on our behavioral rhythms is likely modified by chronotype. Our aim in this study was to identify and characterize differences in sleep and rest-activity rhythms (RAR) between weekends and weekdays and between-chronotypes. Male volunteers (n = 24, mean age = 23.46 y) were actigraphically monitored for 4 weeks to derive objective behavioral measures of sleep and RARs. Chronotype was assessed through self-report on the Morningness-Eveningness Questionnaire. Sleep characteristics were derived using Actiware; daily rest-activity rhythms were modeled using a basic 3-parameter cosinor function. We observed that both Eveningness and Morningness Chronotypes were more active and slept later on the weekends than on weekdays. Significant between-chronotype differences in sleep timing and duration were observed within individual days of the week, especially during transitions between weekends and the workweek. Moreover, chronotypes significantly varied in their weekly rhythms: e.g. Morningness Chronotypes generally shifted their sleep duration, timing and quality across work/rest transitions quicker than Eveningness Chronotypes. Although our results should be interpreted with caution due to the limitations of our cosinor model and a homogenous cohort, they reinforce a growing body of evidence that day of the week, chronotype and their interactions must be accounted for in observational studies of human behavior, especially when circadian rhythms are of interest.

KEYWORDS:

• Chronotype
• rest-activity rhythm
• sleep
• actigraphy
• week

Acknowledgements

The authors wish to acknowledge Robert Koehler and Craig Detheridge for their contributions to the processing and initial analysis of actigraphy data, as well as the 24 volunteers who participated in the study.

Declaration of interests statement

The authors declare no conflicts of interest that influence the content of this article. This study was accomplished as a collaborative effort supported by the Neuroscience Research Unit of Pfizer Inc.

Notes

1. Halloween, Thanksgiving, Thanksgiving Break, Last Day of Classes, Fall Final Exam Study Period, Fall Final Exams, Winter Break, Valentine’s Day, President’s Day, Spring Break

2. All between-Day across-Chronotype p values reported in-text are joint p values derived from joint Wald tests conducted across all seven Days. Two-sample Wald tests were also used to compare individual Days to Sunday, the “base” or “reference” Day; their p values are reported in Supplementary Table 4B.

Additional information

Funding

This work was supported by the Pfizer Inc.