No line of work requires more sleep than “professional athlete.”

Roger Federer and LeBron James have said they sleep an average of 12 hours per day, compared to about 7 hours for the average American. Usain Bolt, Venus Williams, Maria Sharapova and Steve Nash sleep up to 10 hours per day. Most NBA players take naps every game day, sometimes for as long as 3 hours.

Sleep is important, equally as important as exercise and nutrition to athletes earning their living off their body’s performance.

That’s why many athletes are now using sleep-tracking technologies to monitor their sleep – not only how much sleep they’re getting, but how good that sleep is.

“I always knew some days I’d feel great and some days I wouldn’t, but I wouldn’t really know why,” says Boston Bruins defenseman Andrew Ference, who now uses the Zeo personal sleep coach every night. “This has really confirmed how I felt some days, as some days I’ll have great sleep scores and some days I won’t.”

Ference wears the Zeo sleep coach around his head every night. The device reads signals sent from Ference's brain without transmitting anything into the body, and its overall power is less than an average smartphone.

Ference receives a grade each morning on the quality of his sleep session, and he uses that information to sleep better the next night. Maybe his sleep was periodically interrupted because he ate something high in acid for dinner that night, or perhaps a long travel day limited his ability to reach REM sleep.

He can use all of this information -- in addition to input from his team’s nutrition experts -- to improve a crucial part of being an athlete that many athletes don’t think about.

Ryan Dungey, the star motocross racer, said he’s noticed his body performing better since his new sponsor, Red Bull, began monitoring everything about his body.

“A lot of it’s monitored from the hours of how much rest we have, how much training, determining how you feel, and you kind of have to adjust,” Dungey says. “In the long-term and in the short-term, you just monitor it day by day.”

Athletes’ bodies are million dollar commodities, and it only makes sense they use technology to make sure they’re getting enough rest and taking proper care of their bodies.

“It’s a fascinating world and we see more and more application and opportunities to put this type of technology in place,” says Darryl Gehly, president of digital research company Roundarch Isobar. “Wearable devices that use sensors to pick up data and provide feedback to athletes and coaches on anything from sleep quality to exertion levels, are set to be the biggest technological innovation that sports has ever seen.”

In general, napping is considered an activity for the lazy. And for everyday people, taking sporadic naps longer than 30 minutes can disrupt nightly sleep and affect metabolism. But for athletes, who eat and exercise a ridiculous amount, napping can play a role in performance.

“If you nap every game day, all those hours add up and it allows you to get through the season better,” Steve Nash told the New York Times last year. “I want to improve at that, so by the end of the year, I feel better.”

The harmful effects of not enough sleep are the same for athletes as anyone else: decrease in stamina, attention, reactivity, strength, attentiveness… all skills necessary to perform at an elite level.

Remember those athletes we mentioned that slept at least 10 hours per night? Tiger Woods, meanwhile, says he sleeps 4-5 hours per night. Could a lack of sleep, in some small way, be contributing to his career’s downward slope over the last half-decade?

“More and more we’re realizing that right sleep affects your performance,” Ference says. “You play 100-plus games with all that travel, and it really adds up. By the time the playoffs come around, you can really tell who took care of themselves and who didn’t.”