“Funny thing happened on the way to the Moon: not much,” wrote the astronaut Gene Cernan in his memoir about life in NASA’s space program.

Space travelers aren’t the only ones killing time. During their free climb of the Dawn Wall in Yosemite National Park in California, Tommy Caldwell and Kevin Jorgeson spent a large part of many days waiting for shade to hit the wall so they could begin to climb. One study of explorers scaling a Himalayan peak found that they spent a mere 4 percent of their time actually climbing.

A yearning for novelty and sensory stimulation may draw people to extreme activities, but these pursuits inevitably require long periods of boredom, the psychologists Emma Barrett and Paul Martin write in “Extreme: Why Some People Thrive at the Limits,” (excerpt) an intriguing examination. Dr. Barrett and Dr. Martin investigate the challenges facing extreme adventurers, and the traits and skills needed to overcome them.

If tedium is an unexpected challenge, sleep deprivation is all too familiar. As they orbit Earth, astronauts witness 16 sunrises every day, a disruption to their circadian rhythms and sleep cycles. (It’s no wonder that sleeping pills are the most commonly used medications in space.) Successful adventurers often adopt a strategy of tactical napping, the authors note, sleeping in short bursts when the opportunity arises.