What are the common misunderstandings about UV rays and how you feel (summer sun and fatigue)?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
"The sun wears me out" and "I feel drained after being outside in summer" are feelings shared by almost everyone who spends a summer in Japan. The effects of ultraviolet (UV) rays and the effects of heat and dehydration tend to get mixed together. This article uses public sources to pull them apart.
Keep the causes separate
| Factor | Effect on the body | Countermeasures |
|---|---|---|
| UV rays | Skin (sunburn, dark spots, skin cancer); eyes (cataracts, pterygium). Effects on immunity have also been noted | Hat, parasol, clothing, sunscreen, sunglasses (Ministry of the Environment manual) |
| Heat (temperature, humidity, radiant heat) | Strain on temperature regulation → fatigue, heatstroke | Fluids and salt, shade, rest, air conditioning (Ministry of the Environment heatstroke prevention) |
| Dehydration | Sluggishness, headache, poor concentration | Small, frequent drinks (1.2 L a day plus what you sweat) |
| Lack of sleep (summer nights) | A hot bedroom means shallow sleep → tiredness the next day | Air conditioning while you sleep (26–28°C / 79–82°F, left on at a gentle setting rather than on a timer) (Sleep Guide) |
| Temperature gap with air conditioning | A general explanation says a gap of 5–7°C or more between indoors and outdoors strains the autonomic nervous system | Carry a layer; ease the setting before going out |
- UV Index: Japanese summers reach 8–10 ("very high"). The Ministry of the Environment's guide is take precautions at 3 or above, and avoid going out in the middle of the day at 8 or above.
- The phrase "UV fatigue" (紫外線疲れ) is in use, but we could not confirm any public source showing that UV rays directly cause whole-body fatigue. The sluggishness and fever after severe skin inflammation (sunburn) are the body's general response to a burn.
- The idea that UV entering the eyes affects the whole body ("sunburn through the eyes") rests on reports at the level of animal experiments. The Ministry of the Environment manual recommends sunglasses to prevent eye disease.
[Verify] Check the UV Index guide against the Ministry of the Environment's *UV Environmental Health Manual 2020*, and heat measures against the Ministry's heatstroke prevention information.
Misunderstanding 1｜Being in the sun tires you because of the UV
The main causes of tiredness are heat and dehydration. UV affects skin and eyes. Take both sets of precautions, separately.
Misunderstanding 2｜With sunscreen on, going out in summer is safe
Sunscreen only protects the skin; it does not prevent heatstroke. Fluids, salt, shade and rest are separate.
Misunderstanding 3｜On a cloudy day neither UV nor heat is a problem
Under cloud, UV is still 50–80% of a clear day (Japan Meteorological Agency). Humid, cloudy days also bring many cases of heatstroke (sweat does not evaporate).
Misunderstanding 4｜Sunglasses prevent eye fatigue
The main purpose of sunglasses is to protect the eyes from UV (preventing cataracts and so on). Reducing glare does relate to tiredness, but it is a different matter from eye strain at a screen (see our article on eye strain).
Misunderstanding 5｜Summer fatigue goes away once autumn comes
Accumulated lack of sleep and dehydration over the summer is what Japanese people call natsubate (夏バテ, summer fatigue; see that article). Getting the bedroom temperature, fluids and meals right during the summer is the foundation.
---
Sources
- Ministry of the Environment, *UV Environmental Health Manual 2020* [紫外線環境保健マニュアル2020]
https://www.env.go.jp/chemi/uv/uv_manual.html
- Ministry of the Environment, *Heatstroke Prevention Information Site* [熱中症予防情報サイト]
https://www.wbgt.env.go.jp/
- Japan Meteorological Agency, *Basic knowledge about ultraviolet rays* [紫外線に関する基礎知識]
https://www.data.jma.go.jp/gmd/env/uvhp/3-40uvb.html
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
About this article
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
This is general information. Results vary from person to person and depend on your own conditions and surroundings.
Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.
This article is not meant to diagnose or manage any specific medical condition. If you are worried about your health, see a doctor rather than relying on your own judgement.
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