In 2005, researchers at the University of California, San Diego, began an experiment that would last five years. One by one, they brought 164 study participants to a sleep lab at the U.C. San Diego Medical Center, a room with a sweeping view of the city and the surrounding valley. There, participants underwent polysomnography, the most comprehensive sleep test known to science. A polysomnography machine is an octopus of a medical device: It has scalp sensors to record brain-wave patterns; eye trackers to assess rapid eye movements; breathing sensors that are placed on the nose, mouth, and around the chest; a blood-oxygen sensor for the fingers; and sensors on the legs to track movement. The machine produces a chart—resembling a cross between a musical composition and a seismogram—that traces the brain and body minute by minute through the night.
The San Diego researchers planned to use the polysomnography machine to document slow-wave sleep—the phase of sleep “when it’s really hard to wake you up,” as Tomfohr describes it. Slow-wave sleep is thought to be the most restorative period of sleep, and it’s important to good health: Experiments where people are denied slow-wave sleep on purpose have shown that bodies quickly change for the worse. (One paper, published in the Proceedings of the National Academy of Sciences in 2007, found that study participants who were denied slow-wave sleep for three nights—researchers would sound an alarm in their ears when they entered this sleep phase—became less sensitive to insulin, a precursor to diabetes.)
But it wasn’t just slow-wave sleep in general that interested the researchers; they specifically hoped to compare how blacks and whites experienced slow-wave sleep. And what they found was disturbing. Generally, people are thought to spend 20 percent of their night in slow-wave sleep, and the study’s white participants hit this mark. Black participants, however, spent only about 15 percent of the night in slow-wave sleep.