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.