Good sleep is essential for many reasons. It is important for our bodies to repair and function properly.
Research has also shown that insufficient sleep can lead to cognitive decline, as well as conditions like Alzheimer's disease.
One recent study showed that more is not always better. A paper by Washington University School of Medicine researchers suggests that, just as with too little sleep, too much can also lead to cognitive decline.
The researchers wanted to determine how sleep and cognitive impairment were linked over time. They studied 100 people in their late-70s and followed them for four to five years.
At the time of the study, 88 people didn't show signs of dementia. 12 others showed signs of cognitive impairment (one had mild dementia and one with pre-dementia mild cognitive impairment).
Participants were required to take a variety of neuropsychological and cognitive tests throughout the study to check for signs of cognitive decline.
The scores of these tests were combined to create a single score called the Preclinical Alzheimer Cognitive Composite score (PACC). Higher scores indicate better cognition over time.
The single-electrode EEG (EEG) device was worn on participants' foreheads while they slept for between four and six nights.
Three years after the completion of annual cognitive tests, this was only done once. The EEG enabled researchers to measure brain activity accurately, which allowed them to determine whether someone was asleep and for how long. It also revealed how restful their sleep was.
Although only one time was used to measure sleep, it gave researchers a good idea of the participants' sleep patterns.
Although using an EEG for brain activity measurement can be disruptive to sleep the first night, people become more comfortable with the equipment and sleep returns to normal the next night. It is therefore possible to get a good representation from the second night of sleep.
Researchers also considered other factors that could affect cognitive decline, such as age, genetics and whether someone had signs of beta-amyloid (or tau) which are both associated with dementia.
Researchers concluded that cognitive decline was linked to sleep patterns that were less than 4.5 hours or more than 6.5 hours per night, as well as poor quality sleep.
It was interesting to note that the effects of sleep duration on cognitive function were similar to those of age, which is the most risk factor for cognitive decline.
Good night's rest
Research has shown that a lack of sleep can lead to cognitive decline. One study found that those who report sleep disturbances such as excessive daytime sleepiness or insomnia are at greater risk for developing dementia than those who do not.
Research has also shown that those who sleep less often have more beta-amyloid in the brain. This is a common finding in people with Alzheimer's disease.
Researchers aren't sure why sleep deprivation is associated with cognitive decline. One theory suggests that sleep aids in the elimination of harmful proteins that accumulate during the day.
Beta-amyloid and tau, two proteins that are linked to dementia, may be responsible. Interfering with sleep could cause our brain to be unable to eliminate these proteins.
This is supported by experimental evidence that shows that even one night of sleep loss temporarily increases beta-amyloid levels within the brains of healthy individuals.
It is less clear why prolonged sleep is associated with cognitive decline. Studies in the past have also shown a correlation between sleep deprivation and cognitive performance. However, most studies relied on participants reporting how much they sleep each night. This makes it less reliable than using EEGs to measure brain activity.
The new study adds weight to these findings.
Surprisingly, this study found that optimal sleep duration is significantly shorter than previously suggested.
This is a low number considering that seniors should get seven to eight hours sleep each night.
Perhaps it's not the length of sleep that is important, but rather the quality of sleep. This could make it more likely that you are at greater risk of developing dementia. This study showed that cognitive impairment was exacerbated by having less restorative or slow-wave sleep.
We cannot also tell if prolonged sleep durations can be used to predict cognitive decline.
We can't rule it out that participants who slept for longer than 6.5 hours each night may not have had cognitive problems or brain changes that were indicative of dementia.
Although the researchers tried to account for dementia-related factors in their calculations, it is possible that longer-sleepers may have had pre-existing conditions which could have contributed to their cognitive decline. This could include poor health, low socioeconomic status, and low levels of physical activity.
These factors may all be responsible for the correlation between longer sleep and cognitive decline.
Many factors can have an impact on our sleep quality and cognitive decline. Some factors, such as genetic predisposition, cannot be prevented. However there are many things that we can do to reduce the likelihood of developing dementia.
The researchers in this study suggest that a good sleep duration is between 4.5 to 6.5 hours each night. However, a weekend lie-in is unlikely.
Greg Elder, Senior Lecturer and Associate Director, Northumbria Sleep Research at Northumbria University in Newcastle.
This article was republished by The Conversation under Creative Commons. You can read the original article.