Author lived in an underground bunker for 10 days to study sleep, happiness and health—1 key thing she learned

On her quest to better understand how factors like when we eat and how much sunlight we get affect our ability to feel rested, happy and balanced, Lynne Peeples went 50 feet below ground for 10 days.

Peeples, a science journalist and author of "The Inner Clock: Living in Sync With Our Circadian Rhythms," found an Airbnb in Arkansas that was once a bunker during the Cold War.

"This guy had purchased an old nuclear missile silo from the government and souped it up," she said on an episode of the "Everyday Better with Leah Smart" podcast.

In the bunker, "there was no daylight," she said. Peeples also got permission from the Airbnb host to cover all of the clocks on digital devices with black tape so she would have no idea what time it was down there.

The lights in the bunker were dim and red. "We know that red is the wavelength of light that least affects our circadian rhythms," she said.

For her book, Peeples wanted to explore what would happen to her internal clocks "if I cut myself off from those cues that they need to tell time."

During her stay, Peeples documented her experience via voice recordings with the plan to use the timestamps to see how closely she aligned with her typical schedule — like when she ate breakfast or when she went to sleep.

"For the first couple days, it was miraculous because I could look later at timestamps from voice recordings that I made, and I was pretty accurate in living a 24-hour day," she said. "Our clocks inside of us do keep pretty good time."

But about halfway into the experience, Peeples began to feel "really out of sorts" which she likened to major jet lag.

"At the 'worst,' I was completely flipped. I was living my day when everybody else above ground was sleeping. So, I was close to 12