Quick Answer: Core body temperature follows a daily rhythm, dipping to its lowest in the early hours before waking and peaking in the late afternoon to early evening. You start an evening session from a core temperature roughly 0.5 to 1C higher, so you cross the sweating threshold sooner and sweat more for the same work. Morning sessions feel drier and slightly harder to warm up, and neither is a fitness signal.

6am: barely damp after twenty minutes.

6pm: soaked in ten.

Same session, same person, different point on your own temperature curve.

What actually sets your daily temperature curve?

Six factors, and the first is the largest by far.

Your circadian rhythm sets the baseline. Core temperature reaches its minimum roughly two hours before your usual wake time and its maximum in the late afternoon to early evening. The total swing is around 0.5 to 1C in most people — small in absolute terms and large in terms of how close you start to your sweating threshold.

Sweating starts at a threshold, not gradually. Your body begins sweating when core temperature crosses a set point. Starting warmer means crossing that threshold with less exercise, which is why an evening session produces sweat earlier in the workout as well as more of it overall.

Skin blood flow follows the same curve. Evening brings greater peripheral blood flow, which is why hands and feet feel warmer in the evening and why heat dumps more readily then.

Hydration status differs enormously. You wake mildly dehydrated after a night without fluid. Reduced plasma volume means slightly reduced sweat capacity, which compounds the temperature effect in the same direction.

Ambient conditions track the same pattern. Evening air is usually warmer than early morning air, and in late summer evening humidity is often higher too. Environment and physiology point the same way, which is why the difference feels so pronounced.

Prior activity accumulates. By 6pm you have moved, eaten, commuted and been warm all day. By 6am you have been horizontal for eight hours. Your starting metabolic state is genuinely different.

Does this change how you should train?

Six practical implications, and most of them are about warm-up and fluid.

Warm up longer in the morning. Lower core and muscle temperature means lower tissue extensibility and slower nerve conduction. Add 5 to 10 minutes to a morning warm-up compared with an evening one, and expect the first working set to feel worse than it should.

Expect peak performance in the late afternoon. Strength, power and anaerobic performance measurably peak near the daily temperature maximum, typically late afternoon to early evening. Differences of a few percent are common — meaningful for testing, irrelevant for general training.

Front-load fluid on morning sessions. You start behind. 300 to 500ml on waking, before a morning session, offsets overnight losses and makes the session feel considerably better.

Plan more fluid for evening sessions. You will sweat more, so replacement needs to be larger. A simple check: weigh yourself before and after a session, and replace roughly 1.25 to 1.5 litres per kilogram lost.

Do not compare sessions across times of day. Judging your fitness by how hard a 6am run felt against a 6pm run last week is comparing two different physiological states.

Train at the time you will compete. Your circadian curve shifts toward whatever time you train regularly. Several weeks of morning training genuinely improves morning performance beyond the general training effect.

Seasonal note: as autumn approaches, morning air cools faster than evening air, which widens the gap between the two. A morning session that felt similar to an evening one in July can feel dramatically drier and stiffer by late September, purely from environment.

What does sweat rate actually tell you?

Six things worth understanding about what your sweat is signalling.

More sweat is often a sign of better fitness, not worse. Trained people begin sweating sooner and at a lower core temperature, and produce more sweat overall. It is an adaptation — a more efficient cooling system that switches on earlier.

Sweat rate varies hugely between people. Ranges of 0.5 to over 2 litres per hour are all normal depending on size, fitness, heat acclimatisation and conditions. Comparing your sweat volume to a training partner tells you nothing useful.

Salt losses vary independently of volume. Some people are salty sweaters — visible white marks on clothing or a gritty feel on skin — and lose considerably more sodium at the same sweat volume. If that is you and sessions run over an hour in heat, electrolytes matter more than they do for most people.

Dripping sweat is wasted sweat. Cooling comes from evaporation, not from production. Sweat that drips off has cost you fluid without cooling you, which is why humid conditions are so much harder than dry heat at the same temperature.

Sudden changes are worth noticing. A marked drop in sweating during a hot session, along with confusion, chills or stopping sweating altogether, is a warning sign of heat illness and a reason to stop immediately and cool down. This is general information, not medical advice — see a professional if you experience these symptoms or have any unexplained change in sweating.

Heat acclimatisation takes about 10 to 14 days of regular exposure, and fades within a couple of weeks of stopping. This is why the first hot week of a season is brutal and the third is fine, and why an autumn heat spell after cool weeks feels harder than the same temperature in August.

Myth vs Fact: Sweat and Time of Day

Myth: sweating more means you worked harder. Fact: it largely means you started warmer, or the environment was warmer, or you are better adapted. Effort is measured by pace, load and heart rate, not by towel weight.

Myth: sweating more means burning more fat. Fact: sweat is a cooling mechanism. Weight lost during a session is water and returns as soon as you drink.

Myth: morning workouts burn more fat. Fact: substrate use differs slightly in a fasted state, but total energy balance over days is what changes body composition. The time of day effect is small.

Myth: if you sweat less, you are unfit. Fact: sweating less can mean poor heat acclimatisation, low body mass, cool conditions or dehydration. Fitter people usually sweat more, not less.

Myth: you should always train at your peak time. Fact: the consistent session you actually do beats the theoretically optimal one you skip. A 6am habit outperforms a 5pm intention.

Myth: thirst is a reliable guide. Fact: thirst lags fluid loss, particularly in older adults and in cool weather. Weighing before and after training is far more informative than waiting to feel thirsty.

FAQ: Sweat Rate Questions, Answered

Why do I sweat more in the evening?

Your core temperature peaks in the late afternoon to early evening, so you start closer to your sweating threshold and cross it sooner.

How much does core temperature change through the day?

Roughly 0.5 to 1C, with the minimum about two hours before you normally wake and the maximum in the late afternoon.

Should I warm up differently in the morning?

Yes — add 5 to 10 minutes. Lower muscle temperature means less extensibility and a rougher first set.

How do I work out my own sweat rate?

Weigh yourself before and after a session with minimal clothing. Each kilogram lost is roughly a litre of sweat. Replace at 1.25 to 1.5 litres per kilogram.

Is sweating a lot a good sign?

Generally yes. Trained and heat-acclimatised people sweat earlier and more, which is a better cooling system rather than a worse one.

Why is humid heat so much harder?

Sweat only cools you when it evaporates. In humidity it drips instead, so you lose the fluid without the cooling.

When should I be concerned?

If you stop sweating during hot exercise, or feel confused, chilled or dizzy, stop and cool down immediately. Those are heat illness warning signs.

TL;DR:

• Core temperature swings 0.5 to 1C daily, peaking late afternoon to early evening.
• Starting warmer means you cross the sweating threshold sooner, so evening sessions produce more sweat for the same work.
• Warm up 5 to 10 minutes longer in the morning, and drink 300-500ml on waking.
• Sweating more usually signals better fitness, not harder work.
• Weigh before and after to find your rate, and replace at 1.25-1.5L per kg lost.

This week, weigh yourself before and after one morning and one evening session. The number difference explains the towel difference.