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.

Research shows that during the circadian low, cognitive performance, reaction times, mood, and physical performance can all decline compared to daytime levels. One study found that alertness and performance degrade significantly during the biological night even in individuals who are well-rested beforehand (Wright et al., 2012). This doesn’t mean you can’t move well at 3 a.m., but it helps explain why the dark miles can feel so much harder than they should. For ultra runners, the circadian lo

In this article, I’ll break down how Runna hill sessions work, explain those sometimes-confusing audio prompts, help you choose the right hill, and point you towards useful resources to improve your hill running performance.