Sleeping too less or too much could lead to decline in memory and other mental abilities, finds study
The findings call for monitoring sleep duration, especially among middle-aged and older individuals who get as low as four hours or as high as 10 hours of sleep each night
A new study reinforces the importance of getting about seven hours of sleep for a healthy brain. According to the findings, sleeping in extremes — either in insufficient or excessive levels — is linked to a decline in memory and other mental abilities.
The findings call for monitoring sleep duration, especially among middle-aged and older individuals who get as low as four hours or as high as 10 hours of sleep each night, the researchers wrote in their study. "Future studies are needed to examine the mechanisms of the association between sleep duration and cognitive decline."
The link between sleep duration and cognitive function is not new. A decline in mental abilities is considered a risk factor for dementia. People who sleep for more than nine hours a night have a higher risk of both dementia and Alzheimer's, according to SleepFoundation.org. On the other hand, sleep deprivation may also take a toll on the brain. One study suggested that losing one night's sleep could increase the levels of a protein in the brain associated with impaired brain function and Alzheimer's disease.
But a lot about the link between sleep and dementia is unclear due to inconsistent results. "Some cohort studies observed sleep duration to be statistically significantly associated with cognitive decline and incident dementia, whereas other studies did not reach this conclusion," Dr Wuxiang Xie from Peking University First Hospital in Beijing and his colleagues wrote in their study.
The inconsistent results, according to the team, could be down to studies enrolling fewer participants. "In addition, the magnitude of the association between sleep duration, especially extreme sleep duration, and various cognitive domains is unclear," they added.
So the team set out to fill the gaps in their large study. They analyzed data from 20,065 participants, who answered questions on the number of hours of sleep logged every night. The participants were then divided into seven groups based on the number of hours of sleep — from 4 to 10 hours per night — to study its links with a cognitive decline.
The analysis showed that individuals sleeping in extremes — 8 hours and more or 4 and less each night — fared poorly in cognitive tests than those who registered seven hours of sleep. "Memory impairment is the core symptom of dementia and can be considered a factor in the conversion from mild cognitive impairment to dementia," the team explained.
One major limitation of the study is that it is observational, which means the findings do not prove that inadequate or excessive sleep causes cognitive decline. "Furthermore, individuals with memory impairment might not accurately remember their sleep duration," they said.
The researchers are unsure of how sleep could cause a decline in mental abilities. Some scientists suspect that the thinning of a region of the brain called the cerebral cortex could be responsible as they observed this feature in people who sleep in extremes. Alternatively, experts think the immune system has some connection with cognitive impairment. Future studies could determine the mechanism of action, the researchers said.
The study is published in JAMA Network Open.