Just getting enough sleep isn't enough to get all of sleep's benefits, according to Stanford University researchers who studied mice to conclude that those who toss and turn or otherwise suffer interrupted sleep may not be storing memories the way they're supposed to.
"Regardless of the total amount of sleep, a minimal unit of uninterrupted sleep is crucial for memory consolidation," the researchers write in the study, published online Monday in the Proceedings of the National Academy of Sciences.
Determining the effects of various aspects of sleep has been somewhat difficult in the past, they noted. For instance, you can interrupt the sleep of mice, which waken easily, with gentle handling. But because they are sensitive to such awakenings, they stress. And stress itself can impair memory, so it has been hard to say that sleep interruption itself and not the stress is what keeps the brain from storing memories properly.
A summary of the study from Stanford said that "it hasn't been possible" to tease out the impact on memory of specific characteristics of sleep, such as duration, intensity, quality, how much is rapid eye movement and how much isn't, etc. But they found a way to fragment sleep without creating stress or lessening sleep intensity or duration through use of optogenetics.
They describe optogenetics as a "technique in which specific cells can be genetically engineered to be controlled by pulses of visible light." Because it is an experimental genetic modification of brain cells, it's not allowed for human studies. But in the mice, they stimulated neurons that help control changing from sleep to wake states using 10-second light bursts to interrupt sleep without impacting total time, quality or composition of sleep.
Co-lead researcher Luis de Lecea, associate professor of psychiatry and behavioral sciences, called the method "a very fine, very subtle way of sleep fragmentation."
Then they placed each rodent subject in a box with an object to which it had been exposed and one that was completely new to it. Typically, a mouse plays with new objects. But the mice who had experienced fragmented sleep explored both objects about equally, indicating they did not remember having encountered the "familiar" object before. Control mice that didn't have fragmented sleep behaved as one would expect, ignoring the familiar object in favor of the one they'd never seen before.
That, the researchers concluded, meant that the interrupted sleep had a negative effect on memory, pointing to the importance of sleep continuity as a memory builder.
"While the study does not reach any conclusions about the amount of sleep needed to avoid memory impairment in humans, it does suggest that memory difficulties in people with apnea and other sleep disorders are likely connected to the compromised continuity of sleep caused by such conditions," they said.
The results emphasize that sleep is a process, Paul Shaw, a neuroscientist at Washington University in St. Louis who was not involved in the study, told Science News. "Whatever biological function sleep serves takes time. So if you wake up, you disrupt that process and have to start from scratch again."
The research indicates why people who have certain psychiatric and neurological conditions such as sleep apnea and alcoholism that affect sleep continuity without affecting total sleep or sleep quality often have memory problems, according to an article in Medical News Today. Well-rested mice are very curious and playful, Rolls told USA Today. After a night of interrupted sleep, though, she said they "resembled a group of hung-over drunks with no memory of the previous night's debauch." She noted it is similar to how she felt in the first few weeks of her newborns' lives, when they were waking her every hour or so.
It's a well-documented and commented-upon facet of having a new baby. There's even a term coined for it: "Momnesia." In the book, "The Female Brain," author Dr. Louann Brizendine blamed the condition on wildly fluctuating hormones.
But researchers also believe sleep interruptions could contribute, as well.