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You’ve heard that Singapore Airlines resumed, last year, the longest nonstop flight in the world, from its home base to Newark. The flight often takes more than 18 hours, on an Airbus A350. (Of course we’ve got reviews of the experience in business class and premium economy, plus a video on what it was like. There’s no coach class on the flight.)  The direct distance between the SIN and EWR airports is just above 9,500 miles, though the actual flight often goes more than 10,000 to take advantage of winds.

That Singapore flight may lose its title as the world’s longest if Qantas begins to serve New York and London nonstop from Sydney. Those two flights would cover respectively 9,950 and 10,570 miles — at least when there is an aircraft that can fly them nonstop with a full load of passengers, which no jets today can do.

Qantas announced it’s going to begin test flights using a Boeing 787, with just a few people on board. As soon as Airbus or Boeing have a version of their airplanes that can cover those distances without stopping, then it will happen for real, and that will mean 20 hours on a plane.

The trend is clear:  the world’s longest flights are getting longer, and for passengers, that means crazy amounts of time crammed in a metal tube. There could be some dangerous health consequences for travelers who don’t move around at all on those long flights.

But generally there’s no reason to shy away.

A business-class seat on Singapore’s A350 serving the nonstop New York flight (Photo by Zach Honig/The Points Guy)

Jonathan French, a professor in Embry-Riddle Aeronautical University’s Human Factors and Behavioral Neurobiology Department, told TPG in an interview that the biggest health concern for passengers on ultra-long-haul flights is deep vein thrombosis or DVT.

When travelers sit in one place for too long, French explained, blood clots can form, usually in their legs, and could result in a heart attack or stroke. The best way to avoid that potential issue is to move around during the flight. (It can happen on much shorter flights as well.)

Airlines, he said, “should worry about getting those people up and moving periodically so they don’t have a cardiac incident.”

DVT risks aside, French said long-duration flights may be uncomfortable, but they’re not really a problem for your body.

“I don’t think there’s any physiological, within reason, limit that people just can’t take it. People are remarkably resilient, and there is opportunity to sleep and eat and use the restroom, I think we haven’t really hit the limit yet.”

French previously worked as a civilian scientist for the US Air Force, researching best practices to keep flight crews alert and healthy on long-duration bomber missions. In an email, he said an 18-hour flight in a bomber is more difficult on the body than any commercial ultra-long-haul flight, “because the ejection seats are far less comfortable than even coach.”

While French was working with the Air Force, flight crews observed recommended nap schedules on long-distance missions. He said long-distance commercial travelers should follow similar practices if they can.

“There’s volumes written about the importance of naps,” he said. “A three-hour rest is going to provide you with enough rest to where you could be functioning at pretty full levels until your next bedtime.”

French said that your best bet for arriving refreshed to a faraway destination is to fly in a fully-flat bed, but he recognized that that’s not an option for many travelers.

Economy class on an Airbus A350 (Photo by The Points Guy)

“People want to get to their destinations and they’re willing to put up with a lot of uncomfortableness to get there. It could be done better, but they have to be willing to pay for that,” he said. “People will pay, you can pack them in and they’ll still pay to get on that flight.”

On Singapore’s longest flight, the meal service is specially timed and lighting is designed to help passengers adjust their body clocks. French said that while such features are a nice touch on the airline’s part, it’s really difficult to design programs that will help everyone adjust on the same schedule.

“By turning the lights down they’re making it easier for people to fall asleep but harder for people to stay awake who want to. You’re going to be helping some people and hurting others,” he said. “There’s a long way to go for airlines to adapt, but that would be marketable: a way for individuals to adjust their circadian rhythmicity.”

Such individualized options could end up making it easier for passengers in all classes of service to arrive refreshed.

Ultimately though, French said, most travelers just need to find their own best system on super-long flights.

“The smart consumer can handle a very long duration flight,” French said, “especially if they can get that three-to-four hour nap.”

Featured photo by Zach Honig/The Points Guy

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