The variability in circadian phase and amplitude estimates derived from sequential constant routines

Area of Science:

• Chronobiology
• Physiology
• Sleep Science

Background:

• Circadian phase determination is crucial for understanding biological rhythms.
• Constant routine (CR) and dim light melatonin onset are proposed methods for single-cycle phase assessment.
• Limited data exist on the intercycle variability of these phase-setting techniques.

Purpose of the Study:

• To quantify the intercycle variability in circadian phase resolution using both constant routine (CR) and dim light melatonin onset.
• To compare the reliability of core body temperature and urinary melatonin metabolite excretion for phase estimation.
• To evaluate the impact of different analytical techniques on phase variability.

Main Methods:

• Eight healthy males underwent two CRs separated by seven days.
• Circadian phase was assessed via 3-min core body temperature and hourly urinary melatonin metabolite excretion.
• Phase and amplitude were analyzed using cosinor models for temperature and acrophase for melatonin, with variability measured by mean absolute difference between cycles.

Main Results:

• Individual phase estimates varied by 40-60 minutes between CRs, depending on the method and analysis (temperature: 51-62 min; melatonin: 40 min).
• The choice of dependent measure (temperature vs. melatonin) and analytical technique influenced phase estimate variability.
• Group-level phase estimates showed minimal variability (mean algebraic difference < 5 min), indicating high consistency across participants.

Conclusions:

• Constant routine and dim light melatonin onset methods exhibit considerable intraindividual variability in circadian phase estimation over a week.
• Analytical choices significantly impact the observed phase variability.
• Despite individual fluctuations, group-level phase assessments remain highly consistent, suggesting robustness for population studies.