Body Temperature Differences Between Morning and Evening Cycling in Taiwan: A Study of Circadian Rhythm and Environmental Interaction

Based on the Journal of Applied Physiology, Medicine & Science in Sports & Exercise, Sports Medicine, and other international peer-reviewed journals, this article provides an in-depth analysis of the environmental physiology mechanisms behind the “interaction between circadian rhythm and environmental temperature,” and integrates Taiwan’s subtropical climate, mountainous terrain, and local race scenarios to offer evidence-based training and race preparation strategies.

In the field of environmental physiology, the “interaction between circadian rhythm and environmental temperature” is one of the key variables determining whether endurance athletes can perform at their best on a real race course. Many amateur and elite athletes focus their training on power output, pacing, and equipment, yet underestimate the enormous physiological impact of the environment—heat, cold, high altitude, humidity, and air quality. In fact, when two athletes have similar fitness and equipment, the one who better understands how to adjust strategies in response to the environment is often the one who can maintain pace in the latter stages of a race and avoid collapse or accidents. Taiwan’s geographical environment is especially unique: hot and humid summers, cold and damp winters, extreme elevation changes from sea level to Wuling at 3,275 meters, plus urban air pollution and typhoon season—all of which make environmental physiology particularly valuable in the local sports community. This article will take you from cellular and systemic physiological mechanisms, through empirical studies in top international journals, quantitative dose-response relationships, differences in responses across populations, and then to directly applicable training strategies and Taiwan-specific scenarios, finally debunking long-circulated myths—so that your understanding of the “interaction between circadian rhythm and environmental temperature” is truly built on science, not hearsay.

Review of Academic Research

Regarding the scientific exploration of the “interaction between circadian rhythm and environmental temperature,” the field of environmental physiology has accumulated rigorous and rich evidence. Below are several representative studies selected for their value in methodological design, study populations, and strength of conclusions, which together form our current understanding:

1. Drust et al. (2005). Chronobiology International reviewed diurnal variations in core body temperature rhythm and exercise performance.

2. Racinais (2010). Sports Medicine integrated the interactions among circadian rhythm, body temperature, and environment on exercise performance.

3. Chtourou & Souissi (2012). Journal of Strength and Conditioning Research examined the effects of training time of day on performance and adaptation.

4. Facer-Childs & Brandstaetter (2015). Current Biology explored the influence of chronotype on the timing of athletic performance.

Taken together, these studies show that the scientific picture of the “interaction between circadian rhythm and environmental temperature” has been continuously refined as measurement technology advances. Early studies typically manipulated temperature, humidity, or partial pressure of oxygen in environmental chambers, observing changes in maximal oxygen uptake, time to exhaustion, or time-trial performance; subsequent studies introduced ingestible core temperature capsules, near-infrared spectroscopy, stable isotope tracers, muscle biopsies, and molecular markers, allowing us to move from “phenomenological observation” to “mechanistic explanation.” Notably, most high-quality studies employed randomized crossover designs, where each participant served as both experimental and control conditions, greatly reducing the noise of individual differences. However, extrapolation of research conclusions still requires caution: a single environmental variable controlled in the laboratory does not necessarily equate to the complex combination of heat, humidity, wind, radiation, and fatigue on a real race course; nor do responses from well-trained subjects necessarily apply to general amateurs. When interpreting “statistical significance” and “effect size,” we must also distinguish between average trends in the laboratory and practical significance on an individual’s race day—a 3% performance difference may determine rankings in elite competition, while its meaning is relatively limited for recreational riders. It is precisely this careful attention to levels of evidence and scope of applicability that forms the foundation of scientific training.

Core Mechanisms

Human core body temperature is regulated by the circadian clock, exhibiting a rhythm with the lowest point in the early morning and a peak in the late afternoon, with an amplitude of approximately 0.5 to 1°C. This aligns with the diurnal variation in exercise performance: muscular strength, power, anaerobic capacity, and many endurance metrics tend to be better in the afternoon to early evening (near the body temperature peak), because higher body temperature reduces muscle viscosity, accelerates nerve conduction, and increases enzyme activity. In the early morning, lower body temperature, stiffer muscles, and tighter joints require a more thorough warm-up. However, environmental temperature interacts with the circadian rhythm: on Taiwan summer mornings, the environment is cooler and heat dissipation is favorable, making it suitable for high-intensity or long-distance efforts; although the body temperature rhythm favors performance in the evening, summer afternoons to early evenings remain hot with high heat load. Therefore, time-of-day selection is a trade-off between “intrinsic body temperature rhythm” and “external environmental temperature”—in summer, early morning is commonly chosen to avoid heat, while in cooler seasons, the evening takes advantage of the body temperature peak. Individual chronotype can also shift the optimal time window.

To truly understand how the “interaction between circadian rhythm and environmental temperature” affects exercise performance, one must return to the systemic integration of thermoregulation, cardiovascular, respiratory, metabolic, and central nervous systems. Environmental factors translate into measurable performance differences precisely because they act on one or more key links in this physiological chain—whether by limiting oxygen and fuel delivery, disrupting heat dissipation and fluid balance, or altering central perceptions of fatigue and exercise drive. The table below summarizes the key effects of this topic at different physiological levels, helping you build a complete mechanistic picture:

| Physiological Level | Key Mechanisms | Significance for Exercise Performance |

|—|—|—|

| Cardiovascular system | Affects blood flow distribution, stroke volume, and circulatory capacity | Determines oxygen delivery and stability during prolonged exercise |

| Thermoregulation/fluid balance | Regulates heat dissipation pathways, sweat rate, and electrolyte balance | Affects the rate of core temperature rise and dehydration risk |

| Muscle and metabolism | Alters substrate utilization, enzyme activity, and contractile function | Determines sustainable power output and the timing of fatigue onset |

| Central nervous system | Modulates perceived fatigue, exercise drive, and cognitive decision-making | Influences “how hard it feels” and the ability to persist and make safe judgments |

Special emphasis should be placed on the two dimensions of “dose-response” and “temporal dynamics.” The same environmental exposure, under different intensities, durations, and acclimatization states, can produce vastly different or even opposite effects—this is precisely why many popular recommendations are one-sided. The limiting factors of exercise performance also shift dynamically with context: for short-duration, high-intensity efforts, limitations primarily stem from anaerobic energy supply and local metabolism, while for multi-hour endurance events, the focus shifts to the combined effects of rising core temperature, fluid imbalance, glycogen depletion, and central fatigue. The “interaction between circadian rhythm and environmental temperature” deserves in-depth exploration precisely because it can selectively influence certain of these limiting factors. Only by understanding the mechanisms can we determine “how to adjust in a given environment, by how much, and when,” rather than being at the mercy of the environment on race day. The more thoroughly you understand the mechanisms, the more flexibly you can respond across different scenarios such as heat, cold, high altitude, or pollution—and this ability to adapt to context is precisely the dividing line between those who understand environmental physiology and those who train blindly.

Dose-Response Relationship

In environmental physiology, “dose determines effect” is a core principle. Environmental stimuli that are too low fail to reach physiological thresholds, producing no adaptation or impact; excessive exposure may exceed the body’s compensatory capacity, triggering risks such as heat injury, hypothermia, or altitude sickness. The table below summarizes the dose-response correspondence for “circadian rhythm and environmental temperature interactions,” serving as the most important quantitative reference when designing training and race preparation plans:

Dose / Condition	Physiological State	Effects and Key Points
Early morning	Low core temperature, cool environment	Requires adequate warm-up; avoid summer heat
Afternoon–evening	Core temperature peak	Good performance potential
Summer evening	Core temperature favorable but environment hot	Heat load requires trade-off
Individual chronotype differences	Optimal time window shifts	Adjust according to individual needs

As shown in the table above, the effects of environmental exposure often follow a threshold or inverted U-shaped curve: before reaching an effective dose, adaptation or impact increases with dose; but beyond a certain critical point, not only are there no additional benefits, risks and costs rise sharply instead. This means that “finding your own optimal exposure” matters far more than “relentlessly pursuing extremes.” Whether it’s daily heat dose for heat adaptation, altitude and duration for high-altitude training, or exposure time in cold environments, there exists a sweet spot that balances benefit and safety. It is recommended to progressively test responses under different environmental conditions during training (not on race day), recording core temperature or heart rate, perceived exertion, power data, and recovery status to build your own environmental response profile. Remember: laboratory averages are a starting point, not an endpoint; everyone’s body composition, sweat rate, acclimatization level, and genetic background cause individualized shifts in optimal dose. Only by calibrating with your own data can you translate population science into a personal prescription.

Differences Across Populations

The impact of “circadian rhythm and environmental temperature interactions” is not equal for everyone. Age, sex, training status, body composition, acclimatization state, and genetic background all significantly modulate individual responses to the environment. Ignoring these differences and applying a one-size-fits-all recommendation is one of the most common mistakes in applied environmental physiology.

Population Dimension	Response Characteristics	Practical Recommendations
Beginners vs. advanced athletes	Advanced athletes have more mature environmental adaptation and better tolerance, but less marginal adaptation capacity	Beginners should progress conservatively, building baseline tolerance before increasing load
Male vs. female	Differences in body surface area-to-mass ratio, hormonal cycles, and sweat composition	Females should receive individualized assessment of heat dissipation and hydration, and monitor energy availability
Young vs. older	Older individuals have reduced thermoregulation, sweat gland function, and compensatory capacity	Older individuals are more sensitive to extreme environments, requiring greater caution and longer adaptation periods
Body composition differences	Body surface area, body fat, and muscle mass affect heat dissipation and heat production	Larger individuals dissipate heat less easily; smaller individuals cool faster—each carries its own risks

When interpreting individual differences, one must also be wary of a statistical trap: studies typically report “group mean responses,” but beneath the average often lies enormous individual variability. In the same environmental intervention, some may be strong responders, some barely respond, and some may even respond in the opposite direction. This is why even when a study shows “effective on average,” you still need to confirm through your own experimentation which category you fall into. Individualized controlled testing is recommended: across two training sessions under similar conditions, with and without a given environmental strategy (e.g., pre-cooling, heat acclimation), compare power, core temperature, heart rate, and perceived sensation, repeating several times before drawing conclusions.

Take Taiwan’s common amateur endurance population as an example: many are middle-aged cyclists and runners over 35 who train around work commitments. This group’s thermoregulation and recovery capacity are already somewhat inferior to young elites. When facing the humid-hot summer and high-altitude challenges, correct environmental strategies (heat acclimation, individualized hydration, gradual ascent) can yield relatively greater safety and performance benefits. Female athletes, meanwhile, need to pay attention to how the menstrual cycle affects baseline core temperature and fluid regulation, as well as whether energy availability is sufficient. After understanding population differences, you’ll realize: truly professional environmental physiology advice is always an individualized prescription that is “different for each person,” never a one-size-fits-all slogan.

Practical Training Application

Theory must ultimately translate into training plans and race courses. Below is a practical framework for converting “circadian rhythm and environmental temperature interactions” into concrete training and race operations:

• Pre-race environmental rehearsal: All environmental strategies must be rehearsed in training first. “Never try anything new on race day” is an iron rule. Whether heat acclimation, pre-cooling, or hydration pacing, the body needs time to build tolerance and proficiency.

• Periodization mindset: Align environmental adaptation with the training cycle—build tolerance in the base phase, and conduct event-specific environmental acclimation for the target race’s temperature, humidity, or altitude during the pre-competition phase.

• Progressive exposure: Start with mild environmental stimuli, gradually increase load based on bodily responses, and build a personalized exposure dose and timing profile to avoid injury from a single overexposure.

• Objective data monitoring: Combine core temperature (or heart rate drift), power, rating of perceived exertion (RPE), and urine/body weight changes to objectively assess whether environmental strategies are truly effective.

• Holistic context: Environmental adaptation is one component of training, sleep, recovery, and nutrition. No single strategy can compensate for sleep deprivation, dehydration, or flawed training design.

Using a one-week training schedule as an example, different environmental scenarios can be rehearsed across key midweek sessions and weekend long rides: high-intensity days focus on maintaining power quality in the target environment and testing heat dissipation or insulation gear; long-distance days emphasize hydration and sodium pacing, thermoregulation, and long-duration gear tolerance. Through repeated rehearsal, the body can respond to the environment in a near-automatic manner on race day, freeing mental resources for pacing and tactical decisions. The most common mistake many people make is “only seriously confronting the environment on race day while treating daily training casually”—this is precisely putting the cart before the horse. Daily training is the best laboratory for building heat tolerance, testing hydration doses, familiarizing yourself with gear, and cultivating environmental coping rhythms.

It is recommended to integrate an environmental log with your training log, recording temperature, humidity, altitude, wind conditions, environmental strategies employed, bodily responses, and performance data for each key session. Over weeks to months of accumulation, the value of this personalized database will far exceed any generic guide. Additionally, don’t overlook the often-underestimated aspect of “recovery under environmental stress”—high heat or high altitude delays recovery and exacerbates fatigue accumulation. Recovery quality between consecutive training days often determines whether you can steadily accumulate training volume without injury, and training volume is the most fundamental engine of long-term progress. Treat environmental management as a serious part of training rather than a last-minute accessory before race day, and both your progress and safety will be noticeably different.

Local Application in Taiwan

Taiwan’s unique climate, terrain, and race culture add distinctive local color to the application of “circadian rhythm and environmental temperature interactions.” Summer brings high heat and humidity, with apparent temperatures often exceeding 35°C and relative humidity frequently above 80%, making evaporative cooling inefficient and sweat and electrolyte losses far greater than the research scenarios of temperate countries. This means recommendations from foreign literature often need upward adjustment. In winter, the northeast monsoon brings damp cold and strong chilling winds, and mountainous areas may present risks of low temperature and hypothermia. Meanwhile, the extreme elevation gain from sea level to Wuling at 3,275 meters puts high-altitude physiological challenges right at your doorstep.

Taking events such as the Westbound Wuling Challenge, Tour of East Taiwan, Sun Moon Lake Loop, Taroko Marathon, various triathlons, and round-island challenges as examples, athletes should incorporate local and seasonal environmental factors into their planning: summer races should start earlier to avoid high WBGT in the afternoon, and take advantage of the dense convenience store network along routes to enhance fluid and sodium replenishment; high-altitude sections should allow for gradual acclimatization, with attention to cold protection and signs of altitude sickness; winter wet-cold races require stronger windproofing, insulation, and hypothermia protection. By making good use of the Central Weather Administration’s forecasts for apparent temperature, humidity, wind speed, and air quality—converting them into daily environmental risk assessments and strategies—you can balance safety and optimal performance in Taiwan’s variable and demanding environment.

Common Myth-Busting

Myth: “Morning exercise is always more effective.” Core body temperature peaks in the late afternoon, and most explosive and endurance performances are better in the afternoon; the main advantage of early morning is avoiding heat in summer, not physiological performance itself. The choice of timing must weigh circadian temperature rhythms against environmental temperature.

Such myths spread widely because they “sound reasonable,” are easy to pass along by word of mouth, or are amplified by marketing and anecdotal experience. Yet the value of science lies precisely in testing intuition with rigorous evidence: many seemingly obvious environmental notions fail to hold up under controlled experiments in environmental chambers and epidemiological surveys. Environmental physiology is especially rife with oversimplified claims that compress complex dosage, timing, individual differences, and risk into a single slogan. The next time you hear a categorical environmental recommendation, it’s worth asking: “What is the level of evidence for this claim? Who is the target population? Are the dosage, timing, and safety margins clearly defined?” Cultivating this evidence-based critical thinking is more valuable than memorizing any single conclusion, and it is a key step for athletes toward a more scientific approach and avoiding environmental injury.

Conclusion

“The interaction between circadian rhythm and environmental temperature” is a topic in environmental physiology that combines both theoretical depth and practical value. From the international journal evidence reviewed in this article, it is clear that the environment’s impact on athletic performance is real and profound, but by no means unmanageable—the key lies in understanding the mechanisms, mastering the dosage, individualizing adjustments, and coordinating with overall training, recovery, and nutrition. For endurance sports enthusiasts in Taiwan, while grasping the scientific principles, it is even more important to integrate the local hot and humid climate, extreme altitudes, and variable weather, transforming general principles into personalized prescriptions suited to oneself. May every cyclist and runner sweating on Wuling, in the rift valleys, and along the island-circumnavigation route protect themselves, push their limits, and enjoy the purest joy of sport through the wisdom of environmental physiology—amid heat, cold, high altitude, and all kinds of challenges. Before you next step onto the race course, don’t forget: your true opponent is not just the timer, but the entire environment beneath your feet and all around you.

Related Reading

• The Temperature Differential Effect of Night Cycling in Taiwan: A Study on the Impact of Day-Night Temperature Variation on Recovery
• Physiological Responses to Cycling in Cold Environments: Research on Vasoconstriction, Muscle Metabolism, and the Respiratory Tract
• Differences in Athletic Performance Between Maritime and Continental Climates: A Taiwan Case Study
• The Impact of Climate Change on Taiwan’s Cycling Events: A Physiological Prediction Study of Rising Temperatures