Running Through the Night: Understanding Circadian Rhythms and the Circadian Low and the Effects on Ultra Running
- 16 hours ago
- 5 min read
Running Through the Night: Understanding Circadian Rhythms and the Circadian Low
For many ultra runners, night running isn’t just inevitable, it’s part of the adventure. Whether you’re tackling a 50-mile race or pushing through a 100-mile slog, night miles are a test of physical endurance and biological resilience. One of the biggest invisible challenges on those dark stretches is your circadian rhythm, specifically the dreaded circadian low.
If you’ve ever hit a mental wall at 3 a.m., felt your pace drop, or struggled to focus on trail markers in the dark, your body’s internal clock may be to blame.
A note before we start, I am not a medical professional , what follows is my experience, thoughts and where indicated some articles I have uncovered whilst exploring my interest in this topic. I'll talk about the effects of circadian low on ultra running after my own experiences.
What Are Circadian Rhythms?
Your body runs on roughly a 24-hour biological cycle called the circadian rhythm. This internal clock influences sleep–wake cycles, hormone production, body temperature, metabolism, alertness, and reaction times. Light exposure, especially daylight, is the most powerful cue for this rhythm, but feeding, exercise, and social interaction also play roles.
Cortisol (a stress hormone) and melatonin (a sleep hormone) fluctuate predictably across the day: cortisol peaks in the morning to kickstart alertness, while melatonin rises in the evening to prepare the body for sleep. These regular hormonal cycles help sync physical and psychological functions to day and night.
The Circadian Low — What It Is and Why It Matters
Most humans experience a natural dip in alertness and performance, a circadian low, in the early morning hours, typically between 2 a.m. and 6 a.m. This low coincides with the deepest part of the night, when body temperature is at its minimum and sleep pressure is highest.
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 low is more than an inconvenience, it’s a performance barrier.
How the Circadian Low Affects Ultra Runners
1. Decreased Alertness and Reaction Time
During the circadian low, cognitive processing slows. Simple decisions, choosing the best line on a technical section, remembering aid station distances, or responding to environmental cues, become harder. Studies show reaction times are slower and vigilance decreases during night hours, which can increase risk on technical terrain.
2. Impaired Concentration
Ultra racing requires sustained mental focus: pacing, nutrition timing, rhythm, and terrain navigation all demand attention. At 3 a.m., focusing on these details can feel like trying to read in a storm of static. Cognitive fatigue can lead to mistakes, slowing you down even if your legs are willing.
3. Reduced Physical Performance
Physiological measures like muscle strength, power output, and endurance capacity are influenced by circadian rhythms. A meta-analysis of athletic performance across the day showed that peak performance often occurs in the late afternoon or early evening, with a dip in the early morning hours (Teo et al., 2021). For ultra runners logging hours of night running, this biological low point can coincide with key sections of a race.
4. Mood and Motivation
The circadian low doesn’t just affect your body, it affects your mood. Motivation can dip, negative thoughts can creep in, and the effort required to keep moving suddenly feels disproportionate. Psychological resilience matters in ultras, and the night can chip away at it if you’re not prepared.
Recognising the Signs of Circadian Low
To navigate this biological challenge, first learn to recognise it. Common signs include:
Drifting attention or zoning out
Slower reaction times
Increased irritability or emotional swings
Errors in navigation or simple decisions
Sudden heavy-leg sensation despite adequate fuelling
Micro-nodding or feelings of sleepiness even when moving
If several of these start showing up between about 2 a.m. and 5 a.m., it’s likely your circadian rhythm is contributing.
Training Your Body Clock: Practical Strategies
The good news is that with targeted preparation, you can reduce the impact of the circadian low on race day.
1. Simulate Night Running in Training
Regularly run at times that force your body to be alert when it would normally be winding down. Night/time-change training helps:
Desensitise your biological clock
Increase confidence in night running
Improve your physiological adaptation to running when your body expects sleep
Weekly night runs in the weeks leading up to your goal race will decrease the “shock” of night hours.
2. Use Light Strategically
Light powerfully influences circadian rhythms. Exposure to bright light (especially blue-wavelength) can suppress melatonin and enhance alertness. Many ultra runners use:
Headlamps with bright LED settings during night segments
Strategic use of exposure to artificial light at night
Avoiding bright light exposures before planned sleep
In research settings, appropriately timed light has been shown to shift circadian rhythms and improve alertness at night (Czeisler et al., 1999).
3. Nap Smart
Planned naps during multi-day events or long training blocks can help manage sleep pressure. Even 20–30-minute naps boost alertness and cognitive performance. Longer naps (~90 min) can reduce sleep debt more substantially. Timing naps earlier in the afternoon or early evening can reduce sleep pressure for the early night hours.
4. Strategic Caffeine Use
Caffeine can be a powerful ally during the circadian low. It enhances alertness and improves reaction time, especially during sleep-deprived or night conditions. Use it judiciously, a well-timed dose during the night can reduce the performance dip, but avoid overuse that might disrupt sleep later.
5. Consistent Sleep Schedule in Training Block
A stable sleep schedule strengthens your circadian rhythm. Go to bed and wake up at consistent times. This regularity makes it easier for your clock to adapt to occasional night training.
6. Schedule Race-Day Strategies Around Your Biology
If your ultra starts in the afternoon or evening, plan to hit night miles during periods when you’ve trained your body to be more alert. Conversely, minimise decision-intensive tasks or technical segments during the expected circadian low.
Scientific Support for Circadian Considerations in Ultra Running
Although ultra running is a niche athletic pursuit, research on circadian rhythms and physical performance is robust. Key findings include:
Performance rhythms: Athletic performance shows day–night variations, often peaking late afternoon with dips at night. (Teo et al., Journal of Sports Sciences, 2021)
Cognitive performance at night: Alertness and reaction time decrease during biological night even without sleep deprivation. (Wright et al., Journal of Biological Rhythms, 2012)
Light as a circadian modulator: Exposure to timed bright light can shift circadian phase and improve alertness during night hours. (Czeisler et al., Science, 1999)
While ultra racing pushes the boundaries of human performance, these broader physiological findings apply directly to anyone logging long hours on trail during night.
Conclusion: Respect Your Body Clock, Race Smart
The circadian low is one of those invisible race features you can’t avoid, but you can prepare for it. Night running capability is as much about your internal rhythms as your VO2 max. With intentional night training, light management, sleep strategies, and awareness of your body’s signals, you can turn what feels like a biological hurdle into just another part of your ultra running toolkit.
Running through the night doesn’t have to be slow, foggy, or frustrating, with the right preparation, you can own the dark miles.
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