DOI: 10.17605/osf.io/5wu2r
title: Impact of Circadian and Ultradian Rhythms on Metabolic, Cardiovascular, and Athletic Performance Outcomes among Elite Endurance Athletes

description: Rationale: 
The biological rhythms, including circadian and ultradian rhythms, are endogenous oscillations of biological processes that are driven by internal molecular clocks (Liu et al., 2024). However, there is limited research examining how these rhythms interact with training time-of-day, sleep–wake patterns, and recovery dynamics in elite African endurance runners, despite their global athletic dominance.
This project addresses this gap by combining field-based wearable monitoring with a controlled intervention to explore how chronobiological factors, such as training time-of-day and chronotype, affect training quality, cardiovascular regulation, metabolic markers, and overall athletic performance. The findings aim to inform time-of-day–specific training strategies and enhance performance optimization models grounded in sport science and chronophysiology.

Objective: 
The general objective of this study is to investigate the combined effect of circadian rhythm, chronotype, and training quality on metabolic, cardiovascular, and performance adaptations in elite endurance athletes.

Specific Objectives:
• To explore associations between chronotype, time-of-day training, and perceived training quality among elite endurance athletes.
• To analyse the interaction between chronotype, training time (morning vs. afternoon), cardiovascular function and athletic performance outcomes. 
• To determine the effect of training quality on elite athletes performance adaptations (i.e., VO₂max, lactate threshold, and HRV).
• To examine the relationship between prior-night recovery (sleep and HRV) with next-day training and race performance during competition periods. 
• To confirm the combined predictive value of chronotype, circadian rhythm markers, and training quality for elite athletes' performance and recovery outcomes.

Study Design: 
This research employs a mixed-method exploratory–confirmatory design composed of two complementary phases:

1. Exploratory Observational Phase:
• Design: Observational longitudinal field study
• Duration: 12 weeks of real-world field monitoring.
• Participants: 22 elite Ethiopian endurance athletes (Tier 4 & Tier 5) and 8 Coaches
• Data: Continuous wearable-based HR, HRV (RMSSD), sleep, and training load metrics (via COROS, Tymewear, and other validated devices).
• Analysis: Descriptive, correlational, chronotype and rhythm analysis (Cosinor and FFT) to identify individual circadian and ultradian patterns.

2. Confirmatory Experimental Phase (RCT):
• Design: crossover randomized controlled trial comparing morning versus afternoon training conditions.
• Each athlete (22) completes both conditions with 10 – 14 day washouts period.
• Outcomes: VO₂max, HRV, RPE, lactate threshold, and recovery markers.
• Analysis: Linear mixed-effects modeling, repeated-measures ANOVA, and hierarchical testing for rhythm–performance interactions.
• All sessions are supervised by certified coaches, physicians, and physiologists to ensure protocol fidelity and participant safety.

Ethical Approval:
Ethical clearance for this study has been granted by the Institutional Ethical Review Board (IERB) of Addis Ababa University, College of Natural and Computational Science (Approval Reference: EDRE/04/2017/25, dated 10 November 2025).
All participants will provide written informed consent (and parental consent/assent for minors aged 15–17). Data confidentiality, anonymity, and secure storage procedures are maintained in accordance with AAU and international ethical standards.

Expected Outputs:
• Two to three peer-reviewed journal publications derived from the exploratory and confirmatory phases.
• Practical guidelines for training quality and chronobiology-informed training periodization in elite endurance athletes.
• Contributions to the understanding of chronotype and training time-of-day influences on human performance within African athletic populations.

Key Words:
circadian rhythms, ultradian rhythms, heart rate variability, endurance, performance, chronotype, pace, cadence, training districution, training load, training quality, training intensity distribution, wearable technology, training time-of-day, Ethiopia