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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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