This weekend two of the sport's great ultras, Western States in California and Lavaredo in the Dolomites, will send their fields straight into the dark, the latter starting at 11pm and the former finishing through a second night for many runners. Racing in the small hours is one of the defining challenges of ultra-distance running, and the physiology behind it explains why so many races are won and lost not on the climbs but in the quiet, cold hours before dawn.
The body runs on a roughly 24-hour clock, and core temperature follows it, drifting to its lowest point in the pre-dawn window, often somewhere around four or five in the morning. At that circadian low, alertness falls, reaction times lengthen and the rise in melatonin that promotes sleep collides directly with the demand to keep moving. Runners frequently report that the same pace feels markedly harder at 4am than it did at midnight, even when the terrain is easier.
Sleep deprivation compounds the problem. Even a single night without rest measurably degrades decision-making, mood and the ability to judge effort, which is precisely why navigation errors, missed fuelling and poor pacing tend to cluster in the overnight hours. The cumulative fatigue of a long race amplifies these effects, so that the cognitive task of simply staying on course and eating on schedule can become as demanding as the physical one.
Experienced ultrarunners lean on a familiar toolkit to push back. Caffeine, carefully timed, can blunt the worst of the alertness dip; bright headtorches and the arrival of daylight help reset the system; and steady, deliberate fuelling guards against the blood-sugar lows that magnify drowsiness. Many also plan their effort around the dawn, treating the sunrise as a psychological and physiological turning point to be reached intact rather than a milestone to be raced towards recklessly.
Thermoregulation adds a final twist. Night brings cooler air, which can be a relief after a hot afternoon in the canyons or high cols, but it also raises the risk of chilling for runners who have slowed, sweated and stopped eating. The art of the overnight ultra, then, is one of management rather than heroics: stay warm, stay fed, stay awake enough to think clearly, and arrive at first light with the race still there to be run.