On June 14, 2011, Dr. Charles Czeisler stood by the side of a small stage, listening as a colleague introduced him to a crowd of fellow researchers. Just as he prepared to ascend the steps, his cell phone vibrated in his pocket. He’s not sure why he answered it. Maybe he thought it was an emergency; maybe he just wanted to silence it. Either way, he took the call.
From the other end of the line he heard the plea of a team physician for the Boston Bruins, calling from Vancouver. Deadlocked with the hometown Canucks, the hockey team was one game away from its first Stanley Cup title in 39 years. Each team had won every game at home—to win the Cup, the Bruins would need to take away the home-ice advantage.
“What should we do? We need to win this one,” Czeisler recalls the physician asking. He thought it was strange timing, to call the night before Game Seven. He’d never spoken to the Bruins, and he didn’t have time to speak right then. He’d call him back after his lecture.
Czeisler is a tenured professor at Harvard Medical School and a go-to expert for professional sports teams from every major league. In the age of analytics-as-religion, teams are looking for every possible way to squeeze more skill out of elite athletes. They consult experts on everything from the number of minutes a player should be on the court to how many fourth down conversions they should attempt.
But Czeisler recommends something much simpler: more sleep. Professional athletes need the same thing as new parents.
The director of the Division of Sleep Medicine at Harvard, Czeisler is known around the National Basketball Association (NBA) as the Sleep Doctor. Jovial, he presents most of the research with a slight laugh, as if to say none of this should come as a surprise. It’s sleep. And yet, it’s so poorly understood. Beyond sports, he’s also consulted with NASA and the Secret Service.
Czeisler offers the generally accepted advice—naps before games or shifts, seven to nine hours each night. But when he addressed a group of analytics-crazed team officials, sportswriters, and entrepreneurs at the 2014 Sloan Sports Analytics Conference, he said something less expected: It’s the sleep after an event, a lesson, a game, that is most important.
“Interestingly, if you don’t sleep the night after training, then even if you sleep the next night or the next night, you never learn,” he told me in a follow-up interview.
In a 2011 review article on the literature related to sleep, his colleagues Richard Stickgold and Erin Wamsley explained, “During all stages of sleep, the mind and brain are working to process new memories, consolidating them into long-term storage and integrating recently acquired information with past experience.”
Offering a rudimentary explanation of circadian cycles, Czeisler explains that when we sleep, we repeatedly go through 90-to-120-minute cycles. Within those cycles, there are two main types of sleep: deep and rapid-eye-movement (REM). At first, the cycles are dominated by deep sleep, as our brains drain toxins that have built up throughout the day. The brain also flushes out excess synapses—the connections between neurons—that form during waking hours. In the process, the brain consolidates memories and rebuilds energy stores. As the night goes on, the balance shifts, with more active REM dominating the final cycles. Most of the dreams we remember occur during REM.
Solidifying knowledge requires both REM and deep sleep. The first aspect, which happens during deep sleep, is basically a rehearsal: The brain files away the facts, practices the moves learned that day. The second part is integrating those facts and lessons into existing knowledge. This happens during REM.
Stickgold had proven the importance of sleep in integrating new memories in a 2010 study that tested the memories of 60 people. Split into two groups, the subjects learned a set of words, some real and some made up, that sounded similar to common words—like “cathedruke,” similar to cathedral. The first group learned the words in the evening, took a test, slept for the night, and took another test. The tests involved recalling as many words as possible in three minutes—free recall—and recalling words when prompted with the first few sounds—cued recall.
Participants had 10 seconds to recall a word with cued recall. The second group learned the words in the morning, took a test, stayed awake for the day, and then took the second test. The subjects in the first group showed improved free and cued recall after sleeping the night; the subjects in the second group, who stayed awake between tests, showed no measurable improvements in free recall and were worse when asked to recall the words on cue.
Both groups were tested for a third time one week later. This time, the sleeping group showed no improvement from the previous week. The subjects in the second group, though, showed improved recall once they had had a chance to sleep, indicating that sleep is crucial to integrating new words into existing vocabulary.
Czeisler also loves to point to a study conducted at MIT in 2009. By studying the brains of mice as they navigated through a maze, and then as they slept afterwards, researchers were able to determine that the mice were replaying new experiences in their brains as they slept. This replay during sleep proved vital to forming long-term memories and remembering how to navigate the maze.