A new study by researchers at Stanford University, using a technique that manipulates light to control brain cells, has shown that broken sleep causes memory impairment in mice.
Until recently scientists have been unable to separate the effects on the brain of different sleep patterns. But in the newest study, they were able to overcome that problem using the new method, known as optogenetics.
Published in the Proceedings of the National Academy of Sciences (PNAS), the study could help explain memory problems linked to conditions including Alzheimer’s and sleep apnea.
The new study looked at sleep that was fragmented, but not shorter or less intense than normal for the mice. They targeted a type of brain cell that plays a vital role in switching between the states of being asleep and being awake.
Once they had targeted the proper brain cells, the team of researchers sent pulses of light directly into the brains of mice while they slept. This meant they could disrupt their sleep without affecting total sleep time or the equality of composition of sleep.
They then placed the mice in a box with two objects, one of which they had encountered before. Mice would naturally spend more time examining the newer object, and those who had been allowed uninterrupted sleep did just that.
However, the mice with sleep disruptions were equally interested in both objects, suggesting their memories had been affected.
The study shows that “regardless of the total amount of sleep, a minimal unit of uninterrupted sleep is crucial for memory consolidation,” the authors wrote.
Study co-leaders Luis de Lecea, PhD, associate professor of psychiatry and behavioral sciences, and H. Craig Heller, PhD, professor of biology, said: “Sleep continuity is one of the main factors affected in various pathological conditions that impact memory, including Alzheimer’s and other age-related cognitive deficits.”
Experts have long conjectured that sleep is important for memory, but this has been a difficult area to study in the past, mainly because of the sleep-deprivation techniques used in research. Gentle handling is one way to keep animal subjects from sleeping but, as de Lecea explained, “Rodents are very sensitive to physical awakenings. If you wake an animal up it’s going to be up for awhile, and it will experience stress.” And stress itself can affect memory.
The researchers noted that fragmented sleep is also attributed to people who abuse alcohol and those with sleep apnea. But the researchers added there was no evidence of a causal link between sleep disruption and any of these conditions.
“We conclude that regardless of the total amount of sleep or sleep intensity, a minimal unit of uninterrupted sleep is crucial for memory consolidation,” the team added.
The challenge for the researchers was this: “How could they fragment sleep into shorter episodes without affecting sleep intensity or duration and without invoking a stress response, so they could see its effects on memory?”
Knowing that traditional methods of sleep deprivation wouldn’t allow them to do what they needed, the team turned to optogenetics.
De Lecea said the technique represented “a very fine, very subtle way of sleep fragmentation.”
The findings “point to a specific characteristic of sleep “” continuity “” as being critical for memory,” explained Heller.
While the study did not reach any conclusions about the amount of sleep needed to avoid memory impairment in humans, it did suggest that memory difficulties in people with apnea and other sleep disorders are likely connected to compromised continuity of sleep caused by such conditions.
Asya Rolls, PhD, one of the study’s authors, noted that this was just the “first step in looking at one aspect of sleep,” and she and her colleagues plan to further their research into sleep mechanisms used to preserve memory. The team expects other research groups to use optogenetics in animals to manipulate and study different features of sleep.
The researchers also noted that optogenetics cannot be used in humans at this time as it requires still-experimental genetic modifications to brain cells.
Sleep expert Dr. Neil Stanley, a former chairman of the British Sleep Society, told BBC News that during that day people accumulate lots of new memories.
“At some point we have to sort through what’s happened during the day,” he explained, adding that there are “some things that we need to ‘lock down’ as a permanent hard memory.”
“That process occurs in deep sleep. So anything that affects sleep will have an effect on that process to a greater or a lesser extent,” he told BBC News.
Stanley said there was particularly striking evidence that people with sleep apnea had particular problems “locking down” memories.
And people with Alzheimer’s often had trouble sleeping, although, he noted: “There is something there. But whether it’s the degeneration of the brain that causes poor sleep, or poor sleep that aids the degeneration of the brain has not been determined.”
“For patients with the dangerous sleep disorder, obstructive sleep apnea, this study will come as no surprise,” added Miranda Watson, director of communications at the British Lung Foundation.
“Patients regularly stop breathing during the night when their airways become blocked depriving them of a full night’s rest,” she told BBC News. “This interrupted sleep can cause extreme day time tiredness and memory loss.”
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