She lived in an underground bunker for 10 days. Here’s what she learned.

In a Cold War-era bunker-turned-Airbnb situated 50 feet below ground in Arkansas, journalist Lynne Peeples conducted an experiment to better understand her internal clock. She’d always struggled with sleep — growing up in Seattle, the long, dark winters contributed to the effects of seasonal affective disorder (or SAD). From an early age, she says, she understood her circadian rhythms — the body’s measure of time that drives everything from the sleep-wake cycle to appetite — were generally disrupted.

For 10 days, she cordoned herself off from all of the signals the body uses to tell time. She deprived herself of light, from both the sun and electricity, clocks, most screens and other people. (Peeples did have some light: She set all the LED lights in the Airbnb to a dim red light, which wouldn’t impact her circadian rhythms.) She strapped herself up to devices that tracked her temperature, glucose levels, sleep, and heart rate and tried to live as normally as possible. To pass the time, she juggled, played the harmonica, and read on a Kindle. She documented her experience on a typewriter.

About halfway through, her internal clocks had completely flipped. Her stomach was “distant and grumbly,” her chest heavy, and she had a general weak and woozy feeling. Unbeknownst to her, when everyone above ground was sleeping, she was wide awake. Later, while combing through her biometric data with scientists, she discovered her rhythms had fallen out of sync with one another. “I could feel it,” Peeples says. “I was feeling all the symptoms of sleepiness throughout the day, hot and cold at odd times, a little bit of depression. I’m not thinking clearly. My gut was a disaster. All those things happened about the same time.”

Peeples recounts the experience in her book The Inner Clock: Living In Sync With Our Circadian Rhythms, in which she digs into the science of circadian rhythms and explains how understanding our body’s ebbs and flows can help us feel better when we’re awake and asleep. Here, Peeples shares what she’s learned and how you can better calibrate your own circadian rhythms.

This interview has been edited and condensed for clarity.

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