You’ve probably felt it during those pre-dawn runs: that sluggish, heavy-legged feeling where every stride requires extra effort.
Meanwhile, that same pace feels almost effortless when you run after work.
Here’s what most runners don’t realize: research shows [1] that athletic performance can vary by up to 26% throughout the day depending on your circadian rhythm.
That’s not a motivation problem, it’s your biology working against you.
If you’re an adult runner juggling work, family, and training, understanding your body’s internal clock could be the difference between fighting your physiology and working with it. The science is clear: timing matters for performance, adaptation, and even injury risk.
Your Body’s Built-In Performance Window
Your circadian rhythm isn’t just about sleep.
It’s a 24-hour biological clock controlled by the suprachiasmatic nucleus in your brain, and it regulates nearly every physiological process that affects running performance.
Research from the University of Birmingham [2] demonstrated that endurance performance in trained athletes varied significantly throughout the day, with peak performance occurring in the late afternoon and early evening.
Simply put, your body prepares for optimal physical output at specific times whether you realize it or not.
Dr. Thomas Reilly’s work [3] on circadian variation in sports performance showed that most physical performance measures, strength, power output, reaction time, follow the same daily pattern, typically peaking between 4pm and 8pm.
That’s not surprising if you’ve been running for any number of years.
The Temperature Connection
Here’s the likely reason why: your core body temperature follows a predictable daily rhythm, reaching its lowest point around 4-6am and peaking in late afternoon.
And physical performance rises right along with it.
A study published in Current Biology [4] found that every degree increase in core temperature correlates with improved muscle function, enhanced metabolic enzyme activity, and better nerve conduction velocity.
Think of your muscles like a car engine, they simply perform better when warm.
Unfortunately, this means morning runners start at their lowest physiological readiness point of the day.
Your muscles are less pliable, your metabolic systems are just waking up, and your nervous system is operating at reduced efficiency.
The good news is that understanding this lets you adjust your training strategy accordingly.
The Hormonal Symphony
Beyond temperature, key performance hormones fluctuate on predictable schedules throughout the day.
Research by Dr. Juleen Zierath at the Karolinska Institute [5] has shown that morning exercise may be particularly effective for boosting metabolism and fat burning, partly due to elevated cortisol and higher insulin sensitivity.
Cortisol peaks in the morning (between 6am and 10am), mobilizing energy stores and aiding fat metabolism.
This makes early morning runs potentially advantageous for runners focused on fat adaptation or weight management.
However, afternoon and evening hours create a different hormonal environment that’s more conducive to high-intensity work.
Studies examining hormonal patterns [6] show that testosterone remains elevated while cortisol declines in the late afternoon, creating an optimal anabolic environment for strength and power adaptations.
This is why that threshold workout or interval session feels significantly easier at 5pm than at 6am.
Your body is biochemically primed for harder efforts later in the day.
Know Your Chronotype (And Why It Matters)
Now, that doesn’t mean everyone performs identically at the same times.
Research has identified that people fall into distinct chronotypes [7], roughly 25-30% are morning types (“larks”), 45-50% are intermediate, and 20-25% are evening types (“owls”).
Dr. Roland Brandstaetter’s research at the University of Birmingham demonstrated that athletes perform best when tested at their chronotype-aligned time, regardless of their training schedule.
But here’s where it gets interesting.
Recent studies on field hockey players [8] revealed that peak performance occurs approximately 4 hours after waking, potentially more important than the actual clock time.
This time-since-awakening principle means you can partially shift your performance window by manipulating your wake time.
For a 7am race start, waking at 3am positions you at your physiological peak right at the starting line.
The issue is that this strategy requires balancing optimization against sleep quality, don’t sacrifice rest to hit a theoretical performance window.
Training at Different Times: What Actually Works
Let’s look at some practical applications for runners with different schedules.
If you train in the morning (because that’s when life allows), focus on aerobic efforts and easy runs that work with your elevated cortisol rather than fighting against lower body temperature.
An extended warm-up becomes non-negotiable, research shows [9] that proper warm-up can partially compensate for lower morning temperature and reduced muscle readiness.
Pre-run fueling is also more critical for morning runners due to depleted glycogen stores and higher insulin sensitivity.
For evening trainers, this is your window for quality workouts.
Studies consistently demonstrate [10] superior performance in intervals, tempo runs, and strength work during late afternoon and early evening hours.
Your power output, neuromuscular coordination, and injury resilience are all at daily peaks.
The caveat? Finish training 2-3 hours before bedtime to avoid disrupting sleep onset.
Here’s the practical truth for time-constrained runners: train when you can consistently train.
Dr. Karyn Esser’s research [11] has shown that time-specific training creates some adaptation, 6-8 weeks of regular morning training improves morning performance through localized muscular clock adjustments.
The effect is real but modest, and it doesn’t completely override biology.
When You’re Racing Across Time Zones
Travel presents a unique challenge for runners targeting destination races.
Rapid travel across time zones desynchronizes your internal clock from local time, and research shows [12] the circadian system adjusts only about 60 minutes per day eastward and 90 minutes per day westward.
That means full adjustment could take 3-5 days for every time zone crossed.
Dr. Ravi Allada’s analysis of Major League Baseball data [13] demonstrated that eastward travel impaired physical performance enough to erase home-field advantage, with pitchers giving up significantly more home runs after traveling east.
For runners, studies suggest [14] performance impairment from jet lag ranges from 1-10%, potentially costing you several minutes in a marathon.
The most effective strategy is arriving 3-5 days early for major races involving significant time zone changes.
Upon arrival, seek bright light exposure at appropriate times (morning light if you traveled east, evening light if you traveled west) and train at local times to accelerate adaptation.
Unfortunately, complete circadian adaptation takes days many runners don’t have.
Focus on symptom management through strategic sleep aids and caffeine timing when arrival time is limited.
The Bottom Line
Your circadian rhythm creates real, measurable differences in running performance throughout the day.
Research provides clear guidance: late afternoon and early evening represent the physiological sweet spot for most runners, but training consistency at any time beats perfect timing with irregular training.
For most adult runners balancing work and family, life doesn’t always allow optimization, and that’s okay.
What matters most is training consistently at whatever time works for your schedule, using extended warm-ups for early morning sessions, and planning ahead for races involving travel across time zones.
The research is on your side: every run at any time of day makes you a better runner, even if the clock says it’s not “optimal.”
