Too Little Or Too Much Sleep Can Cause Cognitive Decline: The Sleep ‘Sweet Spot’ Identified

Researchers have found a middle range, or 'sweet spot,' for total sleep time, which can help keep people's minds sharp as they age and reduce cognitive decline.

Sleep is essential for our health and wellbeing. But moderation is best in all things including sleep. Too little and too much sleep may affect your cognitive performance, according to a new study published in the journal Brain.

The study, led by researchers at Washington University School of Medicine in St. Louis, found that older adults who sleep short or long experienced greater cognitive decline than those who sleep a moderate amount, even after considering the effects of early Alzheimer's disease.

"Short and long sleep times were associated with worse cognitive performance, perhaps due to insufficient sleep or poor sleep quality," said first author Brendan Lucey, MD, an associate professor of neurology and director of the Washington University Sleep Medicine Center, as quoted by Science Daily.

The study also found a middle range, or 'sweet spot,' for total sleep time where cognitive performance was stable over time. Keep reading to know the sleep 'sweet spot'

Sleep, Alzheimer's and cognitive function

Both poor sleep and Alzheimer's disease are linked to cognitive decline and separating out the effects of each has been a big challenge for researchers. After several years of research, the Washington University team was able to untangle the complicated relationship among sleep, Alzheimer's and cognitive function. They did so by tracking cognitive function in a large group of older adults and analysing it against levels of Alzheimer's-related proteins and measures of brain activity during sleep.

Cognitive decline in older adults is mainly caused by Alzheimer's, which contributes to about 70% of dementia cases. Poor sleep is a common symptom of Alzheimer's which is also known to accelerate the disease's progression. Poor cognitive performance among short and long sleepers was shown in previous studies too, but such studies usually do not include assessments of the effects of Alzheimer's disease on cognition.

In the new study, the researchers analysed sleep and Alzheimer's data of 100 participants whose cognitive function had been monitored for an average of 4 1/2 years. Among them, 88 had no cognitive impairments, 11 were very mildly impaired, and one had mild cognitive impairment. The average age of the volunteers was 75 at the time of the study.

U-shaped relationship between sleep and cognitive decline

The Washington University research team found a U-shaped relationship between sleep and cognitive decline. Cognitive scores declined for participants that slept less than 4.5 or more than 6.5 hours per night -- as measured by EEG but the scores remained stable for those in the middle of the range.

EEG estimates of sleep time are usually about an hour shorter than self-reported sleep time. So, in terms of self-reported sleep, the findings indicate sleeping less than 5.5 or more than 7.5 hours per night, Lucey explained.

The relationship held true even after adjusting for factors that can affect both sleep and cognition, such as age, sex, levels of Alzheimer's proteins, and the presence of the high-risk Alzheimer's genetic variant APOE4.

However, each person's sleep needs are unique, and so people who wake up feeling rested on short or long sleep schedules may not need to change their habits, Lucey stated.

If you're not sleeping well, the researchers want you to know that sleep problems often can be treated.