Why do we sleep?

Why do we sleep? “The functions of sleep are not immediately apparent,” according to Patrick McNamara, an assistant professor of neurology and psychology at the BU School of Medicine and the Boston VA Medical Center. Under a $1.5 million grant from the National Institute of Mental Health, McNamara (CAS’86, MED’91) is studying the evolutionary development of sleep in mammals to help answer basic questions about why we spend a third of our lives unconscious.

There are two mammalian sleep states: REM (rapid eye movement), or “active,” sleep, when dreams occur, and non-REM, or “quiet,” sleep. Scientists do know that non-REM sleep is restorative — and essential to normal neurological function when we’re awake. “But nobody really knows the functions of REM sleep,” he says. “It’s a total mystery. What we’re doing is collecting data on as many mammalian species as we can — so far we’ve got about 120 — and looking at how much time an animal spends in both kinds of sleep. We’re putting it all into a large database to investigate the distinct evolutionary histories and physiological functions of the two sleep states.”

McNamara says that the goal of the three-year study, currently in its second year, is to clarify the role of each sleep state in the restorative function of sleep and illuminate the ways in which breakdowns in REM and non-REM sleep lead to clinical sleep disorders.

He points out that research during the past two decades has produced major advances in understanding sleep within particular species. Furthermore, he says, molecular advances have made it possible to generate phylogenetic, or evolutionary, trees, and new analytical methods provide the tools to examine macroevolutionary change on those trees. “This comparative methodology has entered a golden age,” says McNamara, “but these methods have yet to be applied to questions concerning the evolution of distinctive sleep state functions. We’re using what is known as phylogenetic analysis, which is a set of mathematical techniques that allows you to reconstruct in a step-by-step fashion how each of the sleep states emerged on the tree of life.”

All mammals sleep, but the amount of time they sleep during the course of a day runs the gamut. Giraffes, for example, sleep 1.9 hours, lions snooze for 13.5 hours, and the little brown bat of North America spends about 20 hours a day sleeping. The sleep of these animals — which is studied by observing their behavior and by monitoring the electrical changes in their brain activity — also varies greatly. “Sleep times for a given bout of a particular sleep state also vary considerably across mammalian species, from a low of about 15 minutes to a high of about 2 hours,” says McNamara. Correlating the ways different species sleep with other characteristics — such as anatomy, physiology, and reproductive patterns — can provide clues to the adaptive functions of sleep, he says.

Almost all mammals undergo cyclical alteration between non-REM and REM sleep, with the exceptions of Australia’s spiny anteater, which doesn’t experience REM sleep, and several species of marine mammals. “Dolphins don’t have REM sleep,” he says. “They sleep one hemisphere of their brain at a time,” possibly to control their respiration system, because they are voluntary breathers.

Scientists also know that duration of REM and non-REM sleep states appears to be regulated by separate sets of genes in mammals. For example, says McNamara, mouse cell lines C57BL and C57BR are associated with increased REM and SWS (short wave sleep, or dozing) episodes, while BALB/c is associated with short REM and long non-REM episodes.

He hopes that by developing a Web-accessible comparative database of mammalian sleep patterns, he and other scientists can derive clinical implications from the disturbances in sleep durations of the two sleep states that typically occur in such major human sleep disorders as insomnia and excessive daytime sleepiness. About 70 million Americans have sleep problems, which adds $15.9 billion to the national health-care bill.

McNamara first became interested in the study of sleep when he was an undergraduate psychology major at BU. “I got intrigued by the strange biology of REM sleep,” he says. “It’s associated with vivid dreams, but it’s also really peculiar, because every 90 minutes your brain goes into intense activation during REM sleep and yet your body becomes paralyzed. You can’t move your muscles, and at the same time you’re having what are known as autonomic nervous system storms. There is sexual arousal, emotional arousal, and stress hormones get released. I thought this was really strange. I asked myself, what could possibly be the function of REM sleep?”

With this latest study, he says, he wants to shed light on “an understudied mystery” — the function and evolution of sleep — and facilitate further investigations in the relatively new field of sleep research.