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Lowest perceived exertion in the late morning due to effects of the endogenous circadian system
  1. Saurabh S Thosar1,
  2. Maya Xolal Herzig1,
  3. Sally A Roberts1,
  4. Alec M Berman1,
  5. Noal A Clemons1,
  6. Andrew W McHill1,
  7. Nicole P Bowles1,
  8. Miki Morimoto1,
  9. Matthew P Butler1,
  10. Jonathan S Emens1,2,
  11. Steven A Shea1
  1. 1 Oregon Institute of Occupational Health Sciences, Oregon Health & Science University, Portland, Oregon, USA
  2. 2 Portland VA Medical Center, Portland, Oregon, USA
  1. Correspondence to Dr. Saurabh S Thosar, Oregon Institute of Occupational Health Sciences, Oregon Health & Science University, Portland OR 97239, USA; thosar{at}

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There are daily variations in the rate of perceived exertion (RPE)1 2 during exercise, with lower RPE in the beginning of the night compared with the early morning. We studied whether RPE is affected by the internal circadian system while controlling for any effects of behavioural patterns, including sleep, activity and meals.


Ten healthy adults (six females, aged 52±2 years (mean±SEM)) participated in a forced desynchrony protocol in dim light where all behaviours were evenly spread across the circadian cycle (figure 1A).3 After a normal night of sleep and baseline testing, participants underwent 10 recurring 5-hours 20-min of ‘behavioural cycles’ of 2-hours 40-min of sleep opportunities and 2-hours 40-min of standardised waking episodes.3 Approximately 1 hour after each sleep episode, participants performed mild intensity cycle ergometer exercise for …

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  • Contributors SST had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. MXH, SAR, AMB, NAC, AWM, NPB, MM, MPB, JSE and SAS contributed substantially in the study design or data acquisition and analyses, drafting the manuscript for intellectual content and approving the final version. All authors take accountability for the integrity of this work.

  • Funding Financial support: R01-HL125893 (SAS), F32-HL131308, National Space Biomedical Research Institute through NCC 9-58 and Medical Research Foundation of Oregon (SST), F32DK107146 (AWM), American Sleep Medicine Foundation (MPB), Ford Foundation (NPB) and Oregon Institute of Occupational Health Sciences and CTSA grant (UL1TR000128).

  • Disclaimer The findings and conclusions of this study are those of the authors and do not necessarily reflect the views of Oregon Health & Science University or the National Institutes of Health.

  • Competing interests None declared.

  • Ethics approval Oregon Health & Science University IRB for human subjects research.

  • Provenance and peer review Not commissioned; internally peer reviewed.