The crew headed home aboard the Dragon capsule after their November launch ended years of US reliance on Russia to ferry its astronauts into space.
Four astronauts departed the International Space Station (ISS) late Saturday aboard a SpaceX vessel, after more than 160 days in space.
The capsule, set to land off the Florida coast, would be the first US nighttime splashdown since Apollo 8's crew returned from the moon in 1968.
SpaceX targeted a splashdown around 3 a.m. Sunday. The Coast Guard deployed extra patrols and spotlights.
"The astronauts have removed their spacesuits and are enjoying one more meal in space before returning to Earth. They'll suit up again before the deorbit burn, about 4.5 hours from now," NASA tweeted.
NASA's Mike Hopkins, Victor Glover and Shannon Walker, and Japan's Soichi Noguchi, headed home in the same Dragon capsule that delivered them to the ISS last November.
'See you back on Earth'
"Thanks for your hospitality,'' Hopkins, the spacecraft commander, radioed as the capsule undocked 420 kilometers (260 miles) above Mali. "We'll see you back on Earth."
The high offshore wind kept the four at the ISS beyond their original departure date, Wednesday, allowing Glover to celebrate his 45th birthday in space on Friday.
"Gratitude, wonder, connection. I'm full of and motivated by these feelings on my birthday, as my first mission to space comes to an end,'' Glover said.
Noguchi tweeted a photo of the crew celebrating Glover's birthday on the ISS with "live music, yummy cakes, funny videos [and] lots of laughter."
The four astronauts' replacements arrived a week ago aboard the same Dragon capsule.
Three US astronauts, two Russians, a Japanese and a French were left at the space station after Saturday's undocking.
Last November, the crew were the first to go on a fully operational mission to the ISS aboard a vehicle made by SpaceX.
SpaceX has become NASA's favored commercial transportation partner, ending nine years of dependence on Russia.
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