This may be partly because astronauts don't have to wake up early to see daybreak; they get a sunrise and a sunset every 90 minutes.
"That extra 12 minutes doesn't seem like a lot, but ... after a couple of weeks, you're going to be falling asleep several hours later than you did the first day," Flynn-Evans said.
A key difference, Flynn-Evans said, is that there are more safeguards to catch errors on Earth -- such as hospital computers or nursing staff who might catch a wrong prescription.
"When you're in space, the stakes are much higher," she said. "A missed keystroke can be the difference between life and death, really."
A space oddity
Dinges said the two crewmembers whose sleep remained steady -- even in the absence of conventional daylight -- were those who kept to a strict schedule and found other ways to control their biological clocks.
"The only way to keep your clock entrained to some extent is a regular behavioral routine with daily exercise and eating at the same time," he said. "Why the others couldn't do that, or didn't do it, we're not quite sure."
Out of the blue
Among NASA's gadgets are wristwatches that monitor sleep and light levels, caps that record electric signals in the brain and a robotic arm. The lab also has a driving simulation, which Flynn-Evans hopes will reveal how drivers and pilots will engage with semi-autonomous vehicles that may lead people to "zone out a little bit" when they are tired.
"People aren't very good at determining when they're sleepy," she said.
"Although it may seem like a little thing to have your phone plugged in next to your bed ... that can have a tremendous negative impact on your sleep and your circadian rhythm," she said.
Even with great sleep hygiene, Flynn-Evans said, a looming challenge remains: What happens when we finally make it to Mars?
"We'll have to stay up a little bit later every day," Flynn-Evans said.
There are other obstacles to sleeping in space, Flynn-Evans said, such as the sheer excitement of being on a space mission and the urge to look out at the Earth -- a bright blue light -- right before bed.
"They may just want to look out at the Earth in all its glory," she said. "They may be flying over their hometown."
But it's not all tossing and turning aboard the ISS; some astronauts have found pleasant things about snoozing in space.
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"Floating there, it's actually really pleasurable after a little while," NASA astronaut Sunita Williams previously told CNN's Chief Medical Correspondent Dr. Sanjay Gupta.
Still, the astronaut said she wasn't entirely saddened to leave her