Value Village ／ Health
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 It stays unpublished until legal review and source verification are complete.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.
| 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 |
[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.
The main causes of tiredness are heat and dehydration. UV affects skin and eyes. Take both sets of precautions, separately.
Sunscreen only protects the skin; it does not prevent heatstroke. Fluids, salt, shade and rest are separate.
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).
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).
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.
---
https://www.env.go.jp/chemi/uv/uv_manual.html
https://www.wbgt.env.go.jp/
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.
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.
Tell us if this has changed
Approved contributions keep the contributor's name, shown in five languages.
Other languages：日本語 ／ 简体中文 ／ 繁體中文 ／ 한국어
Value Village is not a government body. Always confirm procedures on the official site.
Written by Value Village from public sources. Nothing here is reproduced from another site.