The habits of the only known hibernating primate, the adorable fat-tailed dwarf lemur, may teach researchers ways to put humans into a low metabolic stupor to survive surgeries and spaceflight, and may one day answer the mystery of why humans need sleep at all.
A hibernating animal may look like it's sleeping, but the two states differ. During hibernators' dormant phase, their breathing, heart rates and metabolisms slow down to a state of "torpor," which doesn't happen during nightly sleep or naps. In fact, most hibernators interrupt their hibernating to squeeze in some sleep.
Why? Studies over the last few years have suggested that the brain uses non-REM sleep time to recharge its batteries. New evidence from sleeping patterns in hibernating lemurs published in PLOS ONE this week strengthens the link between a chemical recharge in brain cells and the need for sleep to survive. On the flip side, since the metabolism is slowed in hibernators, researchers are starting to think they don't need as much sleep.
As primates ourselves, humans are genetically similar to lemurs, and we've shared some of the same evolutionary tracks. So, when hibernation was first discovered in the four species of fat-tailed lemur, scientists guessed that it would prove a better model system on which to study sleep, and perhaps even mimic torpor in humans.
"If we can one day induce torpor in humans — not an impossibility — it would enormously benefit surgical procedures, spaceflight, and certain disease states," Peter Klopfer, one of the authors of the study wrote in an email to NBC News.
The Duke University researchers tracked the EEG patterns of five hibernating lemurs in the wild, and one animal in captivity at the Duke Lemur Center. They noticed that the hibernating patterns of this mammal are different from those seen in rodents like the ground squirrel.
Work with ground squirrels has shown scientists that those cold-weather hibernators reactivate their metabolism every ten days or so during hibernation, during which their brain pattens indicate they are undergoing deep, non-REM sleep (a much more vigorous activity than hibernation).
But hibernating lemurs, when watched over five days, stayed sleepless. It's likely that during the span of their 3-month hibernating phase, lemurs swapped out the need for sleep by sending their metabolism into low gear. So, while hibernating, the animals don't need sleep at all, explained Andrew Krystal, one of the study's researchers. "This is the best evidence that dropping metabolism decreases the need for sleep," Krystal told NBC News.
In other words, if deep sleep is needed to recharge the brain, it makes sense that when the usual chorus of biochemical reactions is slowed to a whisper in hibernating lemurs, and they don't need to sleep to help restore it. As Krystal said, the lemurs aren't "running down the batteries."
As to why the fat-tailed dwarf lemurs can hibernate while other primates can't — Krystal and Co. haven't found an answer to that yet. Since lemurs are our genetic relatives, "it now seems likely that we have the genes that hibernators have — it is just that those needed to hibernate are not turned on or turned off to allow this to happen," Krystal explains. In next steps, Krystal hopes to find out what those genetic players are, and how, potentially, to apply them to human biology.
The authors of "The Relationship of Sleep with Temperature and Metabolic Rate in a Hibernating Primate" are Andrew Krystal, Bobby Schopler, Susanne Kobbe, Cathy Williams, Hajanirina Rakatondrainibe, Anne Yoder and Peter Klopfer.