Can wearing more actually keep you cooler? SKILD explains the science of sun, skin and performance

You’re lined up at the start. The sun is already high, the water is glittering, and you know you’ll spend the next hour – or the next five – with no shade at all.

Your kit suddenly becomes part of the race plan.

A short-sleeve top feels like the obvious choice. Less fabric, more air, more freedom. When it’s hot, who wants more clothing?

But what actually happens in your body when you paddle under direct sun? And could what you wear change that in ways you wouldn’t expect?

We asked Christoph Renger, co-founder of Singapore-based performance-wear brand SKILD, to take us through the science. SKILD makes paddling apparel; the research below is independent.

How does sunlight affect performance?

Most of us default to short sleeves, a singlet or no shirt when the sun is out. Less fabric, less heat – it feels obvious. The research suggests that in direct sun, it’s often the other way round.

One well-controlled study tested exactly this. Eight healthy young men cycled at 70% of their maximum aerobic capacity in a 30 °C room. The researchers changed only one thing: the amount of simulated sunlight/radiation, from full sun down to none.

Full sun: they lasted for 23 minutes. No sun: they lasted 46 minutes. Same temperature, same humidity, same effort – half the endurance, from sunlight alone.

Not air temperature. Not humidity. Sunlight.

A race won’t be twice as slow – tests to exhaustion exaggerate differences – but the direction is clear. And few of us plan for the sun the way we plan our hydration.

What is happening in the body?

It’s more specific than “the sun makes you hot”.

In the study, core temperature, sweat loss, heart rate and skin blood flow were the same in every condition. What changed was the skin. More sun meant hotter skin – and a smaller gap between core and skin temperature: 3.6 °C in full sun versus 5.2 °C in the shade.

Think of your blood as a delivery van carrying heat from your core to your skin, where sweat gets rid of it. The bigger the temperature gap, the more heat each van can drop off. When the sun heats your skin from the outside, the gap shrinks – and every litre of blood carries away less heat.

Research in the Journal of Applied Physiology shows aerobic performance starts to suffer once mean skin temperature rises above about 35 °C. To keep cooling, your body has to send more blood to the skin – in severe heat, up to 7 litres a minute. That blood competes with the muscles driving your stroke, and your heart works harder.

The result: power drops, every stroke feels harder and your pace fades – even while your core temperature still looks normal.

Bare arms in direct sun can reach skin temperatures well above this threshold.

Can covering up really help?

Yes – with the right fabric. A good long sleeve does two jobs at once:

1. It blocks the sun’s radiant heat before it reaches your skin.
2. It keeps evaporative cooling working by wicking sweat to the outside of the fabric, where it can dry.

So covered skin in direct sun can be cooler than bare skin. Field tests by sportswear brands on the Ironman course in Kona, Hawaii, have reported lower skin temperatures under light, wicking sleeves than on bare arms. Even a small drop can decide whether your blood works for your muscles or for your skin.

“But I overheat in long sleeves.”

That’s the most common objection – and usually the fabric is to blame. A heavy, high-GSM (grams per square metre), tight-knit or woven top that holds sweat against your skin can be worse than bare skin.

Three things make the difference:

• Fabric: light, open, fast-wicking mesh.
• Fit: relaxed, not compression, so air can move underneath.
• Water: dip or splash your sleeves. Wet, breathable fabric works like a cold towel you wear – and on the water, you never run out.

What to look for in a sun top

• Light – around 100–125 g per m²
• Open, breathable mesh
• Moisture-wicking and quick-drying
• High water-vapour permeability – lets sweat vapour escape
• Certified UPF 50+
• Relaxed fit, high collar, sleeves long enough to cover your wrists

Why does this matter so much for paddlers?

Paddlers take one of the biggest sun doses in endurance sport:

• No shade – hours on open water, often around midday.
• Reflected UV – the water bounces extra UV back up at you.
• Sunscreen doesn’t stay put – spray, sweat and towelling strip it away.
• Wet skin burns faster – sweating lowers the UV dose needed to burn.

The performance effect is probably smaller in trained, heat-adapted paddlers than in a lab study – heat adaptation helps – but fitness alone doesn’t remove it.

What about the long term?

Sunburn – even mild sunburn – reduces your skin’s ability to shed heat, and it is a recognised risk factor for exertional heat illness.

Over a season, UV adds up. Reviews of outdoor athletes consistently find doses well above recommended limits, and a higher risk of skin cancer – water sports included. Professional cyclists in one race measured daily doses around 30 times the occupational limit.

So it’s also about staying on the water: every burn can cost you sessions, and years of exposure raise your risk of skin cancer.

UPF 50+ fabric blocks at least 98% of UVA and UVB. It doesn’t wash off and needs no reapplying – it’s the most reliable sun protection you can wear. Use sunscreen on everything the shirt doesn’t cover.”

So what should we wear?

Choose your top the way you choose your nutrition or your taper – as a performance decision, not a habit.

Try it yourself

Pick two sunny sessions with similar conditions and the same workout. Paddle one in short sleeves and one in a light long sleeve.

Afterwards, note your:

• average heart rate;
• perceived effort on a scale of 1–10;
• skin temperature or how your skin looks and feels that evening.

Let your own numbers decide.

Your 3-question kit check before every session

1. Will I be in direct sun – and for how long?
2. Is my top light, breathable and quick-drying?
3. Have I put sunscreen on everything the shirt doesn’t cover – face, ears, neck and hands?

About SKILD

SKILD is a Singapore-based performance-wear brand focused on technical apparel for paddlers and athletes training and racing outdoors.

Its range includes lightweight, certified UPF 50+ paddling apparel designed with breathability, moisture management and sun protection in mind. SKILDPRO adds a seamless graphene-printed pattern designed to help spread heat across the fabric so it can dissipate faster.

Learn more about SKILD:

Website: skildwear.com
Instagram: @skildwear

The research discussed in this article is independent of SKILD. The article was prepared based on research and information provided by SKILD.