Case Study: My 24-Hour Treadmill Ultra and the 2–6 a.m. Performance Drop
- 4 days ago
- 2 min read
Case Study: My 24-Hour Treadmill Ultra and the 2–6 a.m. Performance Drop
A two part blog post on circadian rhythms and low. Read the second part, link at the bottom.
During my own 24-hour treadmill ultra, the clearest demonstration of the circadian low occurred between 2 a.m. and 6 a.m. Despite stable pacing, consistent fuelling, and controlled environmental conditions, this four-hour window produced the most pronounced drop in both perceived exertion and cognitive sharpness.
Up to midnight, fatigue was predictable and proportional to workload. However, shortly after 2 a.m., the nature of the fatigue changed. This was not localised muscular breakdown. It was systemic.
One of the most noticeable changes was a drop in core temperature sensation. Even indoors, I felt chilled — consistent with research showing that core body temperature reaches its nadir in the early morning hours. As core temperature declines, neuromuscular efficiency and metabolic output can be impaired, contributing to reduced power output and increased perceived effort.
Objectively, the treadmill speed remained unchanged. Subjectively, the cost of maintaining that pace increased. Stride felt less fluid. Cadence required more conscious control. Effort-to-output ratio shifted unfavourably — a classic marker of circadian influence rather than mechanical fatigue.
Cognitively, the decline was even more pronounced.
Concentration narrowed. Processing simple data, elapsed time, projected distance, fuelling intervals, required deliberate effort. This aligns with research demonstrating reduced vigilance, slower reaction times, and impaired executive function during the biological night, even in the absence of acute sleep deprivation.
Perceived time also distorted. The 3–5 a.m. window felt disproportionately long compared to earlier hours, a phenomenon commonly associated with reduced cortical arousal and increased sleep pressure.
Importantly, this period felt harder than later stages of the event when muscular fatigue was objectively greater. By mid-morning, despite accumulating more mechanical stress, perceived exertion improved. Mood stabilised. Coordination returned. Alertness increased.
The only major variable that shifted was circadian phase.
That experience reinforced a key performance principle: the circadian low can suppress output independently of muscular damage or glycogen depletion. It is a centrally mediated dip in alertness, thermoregulation, and neuromuscular efficiency.
From a performance standpoint, the 2–6 a.m. window represented the highest psychological load and the lowest physiological readiness, despite adequate preparation.
Understanding this distinction matters. If an athlete misinterprets circadian suppression as catastrophic fatigue, pacing decisions can become overly conservative or emotionally driven. If recognised as a predictable biological trough, it becomes a phase to manage rather than a signal to abandon performance goals.
In a 24-hour event, the night is not just about endurance capacity, it is about managing circadian biology under load.
Read Part 2 of the blog, a deeper dive into what I've learnt about Circadian Rhythms and Low
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