A study of 13 countries has shown that those living in the United Kingdom are the most exhausted, with more than a third of Brits (37%) feeling that they do not get enough sleep.

Americans are the fourth worst sleepers on the list, following Ireland and Canada, with 31% feeling under-slept. France, Turkey, Indonesia, China and Spain are among the countries that are better slept, suggesting that insomnia is more common in the English-speaking nations.

The survey, which was conducted by insurance company Aviva, also revealed that only half (51%) of parents say they make sure their children sleep enough and that 44% of British adults feel too tired to exercise. Women are more affected by this than men, with 52% of females feeling they are too tired to exercise, compared to 35% of males.

But why are Brits in particular so under-slept compared to other nations?

Dr Neil Stanley, an independent freelance sleep expert for over 34 years, suggested it might be a cultural thing. “One reason why the U.K. has such a problem with sleep is because we’ve created a 24-hour society more than any country in Europe,” he explained.

“We have overnight television, supermarkets like Tesco are open all night and 10-15 years ago our government passed a law saying pubs could open for 24 hours a day. This is in stark contrast to Paris, which has been closing down music clubs in residential areas, Switzerland – where it’s hard to get a meal past 10pm and it’s forbidden to flush the toilet between midnight and 6am in some neighborhoods, and places like Germany and Austria, where shops close early.”

According to Dr Stanley, having such a 24-hour society makes it harder for Brits to switch off at night. “We feel we should be functioning 24-hours a day, but we’re not a 24 hour country – it gets dark and cold here,” he said. He compared the English tendency to work long, late hours with the out-of-hours restrictions imposed by German firm Volkswagen, which prevents staff from sending emails from half an hour after the end of the working day.

“If you’re paid 40 hours a week you should work 40 hours a week, but we are always connected – even though last thing you should go before bed is work,” he said. “The essence of the problem is that Brits see sleep as disposable – as the thing to do after you’ve done everything else. There is so much evidence that poor sleep is bad for many aspects of physical, mental and emotional health; the world would be a nicer place if we had more sleep.”

Dr Stanley compared Britain’s 24-hour culture with U.S. cities including L.A. and New York, but added that “if you’re in the Midwest, by 9pm there’s no one in restaurants – they’ll have already had their meal and gone home.” He criticized the presidential candidates’ boasts about not needing much sleep – with Hillary Clinton saying she didn’t need any more naps, and Donald Trump claiming he only needs to sleep for three to four hours a day. “The mindset is that sleep is for wimps,” he said.

Sleep is like height, Dr Stanley explained, and it’s genetically determined. Anywhere between three and 11 hours can be considered normal – it’s all about getting the right amount for you, but you can’t train yourself to survive on less. “If at 11am you’re feeling wide awake then you’re getting enough sleep, but if you’re tired then you’re not,” he added. “It’s that simple.”

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Write to Kate Samuelson at kate.samuelson@time.com.