A Fortune report details the role of sleep and sleep monitoring in the training regimen of Olympic medalist Michael Phelps.

Under Armour shared with Fortune some key data that it helped collect for Phelps as he geared up for Rio. One interesting finding: in the 373 nights leading up the Rio Olympics, Phelps averaged seven hours and 36 minutes of sleep. He aimed to get as close to that target as possible while in Rio as well.

The world’s greatest swimmer only began to consider sleep as an important component of his training back in early 2015, after exiting a rehab program he had entered in the wake of a DUI arrest. At that time, Phelps had a big decision to make: did he want to aim for the Rio Olympics by focusing just on winning slots on relay teams, or did he want to achieve more?

“When we first had a sleep monitoring system available to us, Michael was skeptical,” says Phelps’ trainer Keenan Robinson in an interview with Fortune. “We know sleep is important but until recently, the way to pull objective measures and apply to sports training has been limited.”

Read the full story at www.fortune.com