Why Do We Sleep?

Humans do it in comfy beds. Cows do it standing up, and bats do it upside down. Whales do it with one hemisphere of their brain at a time, and birds do it on the fly.

But why does everything sleep? “It makes no sense,” says Patrick McNamara, an associate professor of neurology and psychiatry at the School of Medicine. “Unlike activities such as sex, foraging, and avoiding predators, the functions of sleep are not immediately apparent.”

It’s an evolutionary puzzle that McNamara is determined to solve. Indeed, humans spend a third of their lives sleeping. And despite countless studies that indicate people who don’t get their zzz’s suffer from memory loss and have slower reflexes, the actual benefits of sleep are, at best, obscure.

To better understand this phenomenon, McNamara has assembled a team of scientists to conduct a three-year study called the Phylogeny of Sleep. Funded by the National Institute of Mental Health, the study examines the sleeping habits of animals to trace the evolution of sleep.

McNamara’s team produced an online searchable database that contains sleep patterns of 127 different mammalian species — and the results are as diverse as the species themselves. For example, some animals, such as the red kangaroo, the giraffe, and the rhesus monkey, sleep less than three hours a day, while others, such as the opossum, the bat, and the South American armadillo, sleep between 19 and 20 hours a day.

The team discovered that environmental influences — such as whether the animal is predator or prey or sleeps alone or in groups — can affect how long an animal sleeps. For example, tigers, which are at the top of the food chain, sleep an average of 12 hours longer than deer, which they hunt. Additionally, genetically related species, such as the donkey and the horse, are more likely to have similar sleep patterns. Such clues, McNamara says, could eventually lead to the revelation of sleep’s primary function.

The project delves deeply into the realm of rapid eye movement (REM) sleep, a stage of sleep where the body becomes paralyzed, the eyes dart back and forth rapidly, the brain and sexual system are highly activated, and dreams occur. REM typically takes place about 90 minutes after a person falls asleep. But for people who suffer from sleeping disorders, REM occurs as quickly as in 25 or 30 minutes. The time to reach REM sleep is also greatly reduced in people who suffer from depression. In fact, half of all sleeping disorders involve REM, McNamara says.

REM occurs in animals, too. And while previous studies concluded that mammals with higher amounts of REM sleep had larger brains, McNamara’s team determined that the animal with the most REM sleep was the platypus, which has a very small brain. In fact, when compared to other mammals, humans exhibited only an average amount of REM.

And even more puzzling, they concluded that the function does not appear to be linked to the amount of whole brain tissue or cognitive processing abiltities, nor does it have any connection to any particular physiological parameter, such as the immune system or metabolic rates.

The group also studied non–rapid eye movement sleep (NREM), or slow-wave sleep, which is the deepest phase of sleep. “When you deprive an animal of REM sleep, it takes a while before you see any dramatic changes in function,” McNamara explains. “But when you deprive an animal of NREM, they exhibit very rapid signs of psychological collapse.”

Such findings have led some sleep scientists to question whether REM is even necessary, but McNamara disagrees. “Slow-wave sleep helps repair the body and primes the immune system, but I believe REM sleep is more important for brain functions,” he says. “And if we can determine the purpose of REM, we just may solve the outstanding mystery of biology: why sleep exists at all.”

But why is McNamara so compelled to unlock this mystery? “About 70 million people suffer from sleeping disorders, adding an estimated $15 billion to the national health-care bill,” he says. “If we can determine what purpose sleep serves, we may very likely learn new treatments for sleeping disorders such as insomnia, narcolepsy, and nightmares.”

McNamara is also involved in a second NIMH-funded sleep study, called REM/NREM Processing Specializations, which explores potential cognitive processing specializations of REM and NREM sleep states.

Vicky Waltz can be reached at vwaltz@bu.edu.