How well we sleep impacts how we think and feel, as well as our alertness, memory, and concentration. “Sleep quality and quantity are directly related to the health of the brain,” says Beth A. Malow, MD, MS, FAAN, professor of neurology and director of the sleep disorders division at Vanderbilt University Medical Center in Nashville. Several studies have demonstrated an association between sleep disturbances such as insomnia, fragmented sleep, sleep apnea, and even excessive napping and an increased risk of cognitive decline over time, says Brendan P. Lucey, MD, assistant professor of neurology and director of the sleep medicine section at Washington University School of Medicine in St. Louis.
In 2009, a series of studies on mice conducted at Washington University were among the first to suggest that chronically sleep-deprived subjects develop higher levels of harmful amyloid beta and tau proteins—considered, along with neurofibrillary tangles, to be hallmarks of Alzheimer’s disease. “Think of tau and amyloid as the waste produced by typical nerve function,” says Charlene Gamaldo, MD, FAAN, associate professor of neurology and medical director of the Johns Hopkins Sleep Disorders Center. “Normally, the brain clears these metabolic waste products away.”
And it may clear away these proteins during sleep, according to a landmark 2013 rodent study in Science by researchers at the University of Rochester Medical Center, who showed that during deep sleep, when neural activity quiets down, cerebrospinal fluid (CSF) bathes the brain, washing away excess amyloid beta and tau proteins. A more recent study, published in the November 2019 issue of iScience, provided further insight into CSF’s function. MRI scans taken while subjects were sleeping showed that during deep sleep, blood flow in the brain diminished as pulsing waves of CSF flushed out excess amyloid beta and tau, presumably girding the brain against cognitive decline. So while it has been known that sleep has some value for survival, these reports seem to put sleep front and center in terms of protecting us from cognitive decline.