Despite years of research and several different theories, it is still unclear why humans sleep. But by observing the hibernation habits of a fat-tailed lemur that is a close genetic relative to humans, researchers may be one step closer to solving the mystery.
Humans spend up to a third of their lives sleeping, and theories as to why this activity occurs range from conserving energy to processing information and memories to removing toxins that build up in the body. But a group of researchers at Duke University studied the hibernation habits of fat-tailed dwarf lemurs and found strong evidence that supports the theory that sleep is connected to regulating metabolic rates and body temperatures in humans.
The team's findings were published in the journal PLOS ONE on Wednesday.
The dwarf lemurs are the closest genetic relatives to humans that also hibernate, and do so for up to seven months each year. They spend that time in a state known as torpor, when their heart rates drop from 120 beats per minute to just 6, they breathe about once every 20 minutes and their body temperatures fluctuate with the outside air.
And as their body temperatures rose, researchers also saw the lemurs slip into a state of sleep known as rapid eye movement, or REM sleep, when most dreaming is believed to occur.
Andrew Krystal, a sleep researcher at Duke University and lead author of the study, says this is a "novel" finding because that type of sleep has never been recorded in animals during hibernation and that it provides strong evidence of a link between sleep and body temperature regulation.
Another significant finding, Krystal says, is that the researchers did not see states of non-REM sleep in the hibernating lemurs. Several previous studies have suggested that a rundown of metabolic "batteries" in the brain drives the need for non-REM sleep. Over the course of the day, energy gets stripped away from cells, producing the need for humans to "recharge" cellular batteries through non-REM sleep, according to Krystal.
But because the lemurs did not show signs of non-REM sleep, it supports the idea that if the metabolic rate of an animal – or a human – is low enough, there is not as much of a need for non-REM sleep.
By better understanding the physiological aspects of hibernation and why humans sleep, scientists may one day be able to induce hibernation-like states in humans, which could benefit heart attack patients or those who have suffered brain injuries and strokes, Krystal says.
In heart attack patients, for example, blood supply to the heart is blocked and a person is unable to get the necessary oxygen to survive.
"If you could decrease the need for oxygen by inducing a hibernation state, there would be no damage," Krystal says.