BRUSSELS—When Europeans set their clocks back next Sunday, it could be one of the last times 500 million people simultaneously get an extra hour of sleep.
After four decades of synchronized time change, the European Union’s 28 countries are debating whether to go their separate ways, risking a temporal Tower of Babel over which clocks where are springing forward or falling back.
Moving clocks back and forth from daylight-saving time every six months is a nuisance, but leaving the decision to each country strikes some as chaos.
The EU has three time zones, but most people live on Central European Time. A post-synchronized EU raises such questions as: Who sets local time? How will it affect shipping times? What time does my plane land? And, from Daniel Dalton, a British Conservative lawmaker in the European Parliament, who asked: “The system isn’t broken, so why fix it?”
With barely 1% of the EU’s population, Finland, it seems, has lobbed a time bomb into the bloc with its push for free-choice clocks. For the Finns and others in the far north, getting the authority to keep standard time, or winter time, all year would provide extra morning light in winter.
Another argument is that the twice-a-year ritual is a chore.
Italian lawmaker Angelo Ciocca scoffed at that. Earlier this year, he held up a wall clock in the European Parliament and showed how easily his finger could push the hour hand. “Here we are crazily talking about a pointless issue that doesn’t interest anyone,” he said, “certainly not Italian citizens.”
Portugal and Greece have already said they wanted to keep the practice of setting clocks an hour ahead in spring and back an hour in fall.
Others have trotted out the dairy cows. Irish studies showed that milk output drops for a month after the seasonal clock changes, because cows are most productive when milked at 12-hour intervals.
Violeta Bulc, the EU’s transport commissioner, acknowledged the problem sounds funny—but, “for cows, it’s hard to understand that they need to be milked one hour earlier or later,” she said.
Last month, European Commission President Jean-Claude Juncker proposed ending mandatory shifts to and from daylight-saving time. EU transport ministers, who will decide what appears a long-shot change, begin talks on Monday.
Politics has framed the debate as a race against the clock. European Union officials want to show they aren’t the power-mad bureaucrats that some allege. Europe’s establishment leaders fear a sweep by nationalist, antiestablishment candidates in elections for the European Parliament in May.
“We are out of time,” Mr. Juncker said, urging governments to agree to ceding EU clock control.
Even if governments miraculously agree, no changes can be expected before 2020. Airlines and logistics companies would need at least 18 months to adjust schedules.
Countries have struggled with time zones since steam trains made coordination across distances necessary more than a century ago. China and India solved the issue by each adopting one nationwide time zone.
It’s not perfect. India sets clocks 30 minutes from most other countries, and neighboring Nepal has a 15-minute time difference from India.
Turkey and about a dozen other countries abandoned seasonal time changes in recent years. Russia, which spans 11 time zones, adopted permanent summer time in 2011, giving more evening light. In 2014, it swung to permanent winter time.
The EU debate is marked largely by geography. At the equator, days and nights remain roughly 12 hours long. For southern Europeans, the day is only about five hours longer in June than in December. In Arctic Europe, the swing is roughly 20 hours, leaving plenty of nighttime to stew about it.
People in the north of Europe overwhelmingly prefer to stay on winter time, according to a European Commission survey that drew nearly five million responses. Southern Europeans liked summer time better.
“I have the feeling in winter that life ends when it gets dark,” said one German in the survey who favored permanent summer time. A Finnish respondent argued in favor of permanent winter time, saying it is “impossible for old people to change the clock on the microwave two times a year.”
“This is something we’ll argue about for a long time,” a Dutch diplomat said.
One European diplomat called ending synchronization “total nonsense,” and clocks risked becoming weaponized. If France and Germany were to feud, the diplomat said, they could threaten to adopt different time zones.
Brexit-watchers fear the island of Ireland could get two zones if the Republic of Ireland, an EU member, and the U.K., which includes Northern Ireland, adopt different settings after the U.K. leaves the EU next year.
Clocks have already marked time in geopolitics this year. North Korea displayed its diplomatic thaw by re-synchronizing clocks with South Korea. For nearly three years, clocks in the two countries had been set a half-hour apart.
Belgian Prime Minister Charles Michel felt he needed to reassure his country, which is already culturally divided between the Dutch-speaking north and the French-speaking south.
“It would be absurd if there were different choices within the Benelux countries,” he said, referring to neighboring Luxembourg and the Netherlands.
Finland recently polled its citizens on which time they preferred. The result: A slim majority, 52%, opted for winter time; 48% for summer time. “People just prefer summertime as a word,” Finnish Transport Minister Anne Berner said.
Till Roenneberg, a professor of chronobiology at Munich University’s Institute for Medical Psychology, said those who opt for summer time may not realize it means darker mornings.
Sticking with perpetual summer time, he said, could increase the risk of depression, diabetes, sleep problems and learning difficulties: “This means we Europeans will become fatter, dumber and grumpier.”
Corrections & Amplifications
The seasonal clock change in Europe is scheduled for next Sunday, Oct. 28. An earlier version of this article said Sunday. Northern Ireland is in the U.K. An earlier version incorrectly stated it was in Great Britain. (Oct. 18, 2018)
Write to Valentina Pop at valentina.pop@wsj.com