Brief disturbances of sleep may interfere with the normal process of solidifying the memories we form during the day.
In a new study, healthy people performed better on a task they had learned the day before after getting a good night's sleep. By contrast, participants with mild sleep apnea, which causes sleep disturbances, showed virtually no improvement on the task the next day.
The finding adds to a growing body of research showing sleep aids in memory consolidation and can improve the ability to perform certain memory tasks, particularly those that require motor memory, such as riding a bike. But the study also shows "it's not just the amount of sleep, it's really having restful sleep that’s not interrupted that is important," said study researcher Dr. Ina Djonlagic, a physician in the division of sleep medicine at Brigham and Women's Hospital in Boston.
It's possible many other sleep disorders besides sleep apnea interfere with sleep's ability to consolidate and enhance memories, Djonlagic said.
She added that the findings provide a reason to treat mild sleep apnea, something physicians may not do if it does not cause daytime sleepiness or fatigue.
"We clearly show there's an impairment: They're impacted," Djonlagic said.
Sleep and memory
Scientists divide the memory process into three main stages: encoding, consolidating and retrieving. The consolidation of memories includes stabilizing and storing them for the long term.
The study involved 15 healthy people and 16 patients with sleep apnea, a disorder characterized by pauses in breathing during sleep that can cause people to briefly awaken. Those with mild sleep apnea awaken about 15 times a night, Djonlagic said.
Study participants learned a motor task in the evening in which they typed the same sequence of five numbers over and over again. The researchers said the healthy and the sleep apnea groups learned the task equally well; both became more accurate and faster at typing the sequence over time.
After a night's rest, and with no additional training, the healthy participants showed a 14 percent improvement in their task accuracy and speed. However, the sleep apnea patients showed almost no improvement, and some did worse than the day before, Djonlagic said.
Alertness was not an issue: Both groups performed equally well on a reaction task in the morning testing their alertness, Djonlagic said.
No relationship was found between participants' performance on the morning task and the amount of oxygen deprivation they experienced at night. However, there was a link between task performance and the number of times participants were roused from sleep: The worst performances appeared related to the greatest number of disturbances.
"This highlights the fact that the arousals that are associated with sleep apnea truly can have an impact on someone's memory," sleep expert Dr. David Neubauer, who was not involved in the study, said of the findings. The impact may be especially pronounced in people with more severe sleep apnea, said Neubauer, who is at Johns Hopkins University School of Medicine.
Sleep apnea classification and treatment are currently determined mainly by looking at the amount of oxygen deprivation patients experience at night, and not at how often they are roused, Djonlagic said. The findings suggest this may need to be reconsidered, she said.
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