People memorize 10 a.m. to 4 p.m. Then 9:50 looks free, and 4:10 looks done.

That six-hour block is a local summer shorthand, written mostly for mid-latitude days. It is not the World Health Organization’s rule. It is not the same six hours in Auckland in January, London in November, or a town that jumped the clock for daylight saving. The UV index travels. The printed hours do not.

Peak hours are not protection hours

Two windows get sold as one.

WHO’s Global Solar UV Index guide puts the daily maximum in the four-hour period around solar noon. Solar noon is when the sun is highest—not when the clock says 12:00. Longitude and daylight saving slide it; WHO puts that between local noon and 2 p.m. The AAD’s public line is close to that peak band: strongest rays 10 a.m. to 2 p.m.

Protection is a wider job. WHO’s line is UV index 3 and above—Moderate, not High. Some countries then issue sun-protection times for every hour the forecast hits 3. That is not a four-hour box.

The U.S. 10-to-4 slogan tries to cover both. EPA: almost half of daytime UVB arrives between 10 a.m. and 4 p.m. CDC’s Yellow Book tells travelers to avoid direct sun in that same block. “Almost half” is the tell. The other half of the day’s UVB still lands outside the slogan.

So even on a U.S. summer day, 10-to-4 is not “when UV exists.” It is a compressed peak-plus-shoulder. Other desks print other compressions:

• Cancer Research UK: strongest hours in the UK 11 a.m. to 3 p.m.; sun strong enough to damage skin mid-March to mid-October
• Cancer Council Australia: seek shade in peak hours, usually 10 a.m. to 4 p.m.—and follow issued sun-protection times whenever UV is 3+
• NIWA / New Zealand: the summer Sun Protection Alert, when clear-sky UVI exceeds 3, runs around 9 a.m. to 6 p.m.

Same sky physics. Four clocks. New Zealand’s summer protect-from-3 band is about nine hours, not six.

The sun’s height writes the hours

UV follows how high the sun sits. WHO is explicit about where the neat midday peak even applies: outside the tropics, the highest levels occur around solar noon in the summer months.

Move the map or the calendar and the window changes shape.

• A stronger summer peak usually means more hours at 3+, not only a sharper noon. New Zealand’s 9-to-6 alert is that shape. Victoria’s SunSmart program usually wants cover from mid-August to the end of April—most of the year, not a school-summer 10-to-4.
• High-latitude winter can close the window. NIWA: New Zealand winter noons often peak at UV 1 or 2. SunSmart Victoria: May to August typically sits below 3 unless you work outdoors, stand on snow, or stay out a long time. McKenzie and colleagues put mid-latitude southern winter doses at less than 5% of summer. A memorized 10-to-4 in that season is covering a peak that never arrives.
• Daylight saving slides the wall clock. Solar noon does not. WHO’s noon-to-2 p.m. range is that slide. A “wait until after 4” habit borrowed from a non-DST poster can dump you under the real peak.

Heat is a third clock. It lags solar noon. That split is UV peaks before the heat. This piece is the other mistake: treating a printed hour-range as portable.

The trap is packing a clock

Three everyday misses:

1. "It's 4:05, so I'm done." EPA’s own fraction leaves about half the day’s UVB outside 10-to-4. A 4 p.m. Moderate hour still writes dose—see hours at UV 4.
2. "I'll wait until 10." On a High or Extreme peak day, UV 3 often arrives before the slogan. WHO already wants cover there. The shadow test will say so while the clock still looks early.
3. "I know the hours from home." Importing 10-to-4 on a southern summer, or keeping it through a northern winter, is the same error as importing a latitude. Same latitude is not the same UV is intensity. This is how long the 3+ band stays open.

A slogan also does not mean UV is zero at 9:59. It does not mean tropical days skip a noon peak—they still have one. It means the edges of the 3+ band are a local fact, not a poster.

Read today’s hours

Use the live UV index for the place you will actually stand:

1. Read current, today’s peak, and the next few hours—not a remembered 10-to-4. Those three numbers are in how to use a live UV index.
2. Treat UV 3 as the start of cover, not 10 a.m. and not High. See what UV index requires sunscreen.
3. Note the peak’s time. If it sits near 1 p.m., daylight saving or longitude has already moved the old noon slogan.
4. If the next few hours stay at 3 or above, keep cover on past 4 o’clock. The slogan’s closing bell is not an instrument.
5. If today’s peak never reaches 3, do not run a summer timetable. Snow and a long outing are the usual exceptions the cancer councils already name.

The homepage hours are for this latitude, this date, this sky. A six-hour rhyme is not.

Sources

EPA’s “almost half” of daytime UVB between 10 a.m. and 4 p.m. is in UV radiation and sun exposure. CDC uses the same clock in the Yellow Book sun-exposure chapter. AAD’s shorter 10 a.m.–2 p.m. peak line is Practice Safe Sun. WHO’s four-hour maximum around solar noon, noon-to-2 p.m. solar-noon range, and protect-from-3 line: UV index Q&A. “Outside the tropics / summer months” for the neat midday peak: Global Solar UV Index: a practical guide (WHO, WMO, UNEP, ICNIRP, 2002). UK 11 a.m.–3 p.m. and mid-March–mid-October: Cancer Research UK. New Zealand summer ~9 a.m.–6 p.m. alert, winter peaks of 1–2, and southern mid-latitude winter dose <5% of summer: McKenzie et al., 2014 NIWA UV workshop and NIWA UV Index Information. Victoria’s mid-August–April season: SunSmart. Those clocks are local public-health shorthand. They are not interchangeable.