Why Spring Makes You Feel Sluggish: The Science of Temperature Gap Fatigue and the “Natural Air Conditioner” You Should Be Wearing
Warm enough during the day to wear just a shirt, yet suddenly cold after sunset. After sweating on a crowded train, your body quickly cools in the outside air. April is actually one of the most difficult seasons of the year for regulating body temperature.
As we age, this temperature gap is no longer just discomfort, but accumulates as "fatigue that remains until the next day". Here, we explain its mechanism and how clothing can solve it.
THE DATA April being “the hardest for temperature regulation” is proven by data
According to meteorological data, Tokyo’s average daily temperature range in April is about 11°C, one of the largest in the year.
The issue is the burden this temperature gap places on the autonomic nervous system. Studies show that temperature differences exceeding 7°C can disrupt autonomic balance. April constantly exceeds this threshold.
Approx. 11°C
One of the largest temperature gaps of the year
7°C+
April frequently exceeds this threshold
Approx. 27 kcal/kg
Heat released when absorbing moisture
Figure 1: Monthly average temperature range (Tokyo). April frequently exceeds the 7°C threshold.
THE ROOT CAUSE The true cause of fatigue is “hidden sweat chill”
One cause of vague spring fatigue is sweat chill. In mountaineering, there is a saying: “Cotton kills.”
Cotton absorbs moisture but takes 30–60 minutes to dry. During that time, evaporation continuously draws heat from the body.
The autonomic nervous system controls temperature, heart rate, and immunity. When body temperature drops suddenly, emergency responses consume large amounts of energy, leading to fatigue that carries into the next day.
Cotton Shirt Case
Sweating → retains moisture → cools due to evaporation → fatigue accumulates (lasting into next day)
Merino Wool Case
Sweating → absorbs moisture at vapor stage → remains dry → generates heat → stable body temperature → less fatigue
The key is not sweating itself, but how the fabric handles moisture afterward.
Figure 2: Difference in sweat chill mechanism by material.
THE MECHANISM Why it is called a “natural air conditioner”
Merino wool has moisture control about 40× polyester and 7× cotton.
Its fiber structure absorbs moisture internally while repelling liquid water, allowing sweat to be handled as vapor.
It also generates heat when absorbing moisture (~27 kcal/kg).
“Cotton kills” is widely known in outdoor fields. Wet cotton loses insulation and increases hypothermia risk.
| Situation | Merino Wool Response | Benefit |
|---|---|---|
| Warm daytime | Releases moisture as vapor | Dry and comfortable |
| Evening cooling | Generates heat | No sweat chill |
| Air-conditioned office | Prevents rapid cooling | Reduces body stress |
THE SOLUTION Leave body condition management to clothing
Instead of relying on willpower, let clothing regulate temperature automatically.
C-one’s Merino wool (18.5μm) is softer than the itch threshold, providing a smooth, silk-like feel.
Prevent temperature fatigue just by wearing it.
Cool when hot, warm when cold. Protect your body from April’s temperature changes.
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