Sleeping too much just as bad as insufficient sleep where cognitive decline is concerned, new study shows
Profound changes in neurological functioning during sleep, as the study shows, have immense effects on mood, memory and cognition.
Sleep is an essential function of life, almost as important as oxygen, water and food for survival. The fact that sleep affects your physical and psychological wellbeing is well known and many studies have underlined how sleep is also linked to cognitive function and abilities.
A study, published in Wiley Interdisciplinary Reviews: Cognitive Science in 2010, explains that sleep plays an important role in the consolidation of different types of memories and contributes to insightful and inferential thinking. Sleep has stages that are physiologically and neurochemically distinct, like rapid eye movement (REM) and non-rapid eye movement (non-REM) sleep. Each of these stages is accompanied by complex changes in the patterns of neural activity and neurotransmitter release, which regulates the release of hormones mediated by the lateral and posterior hypothalamus. These hormones include histamine, norepinephrine, serotonin and acetylcholine.
Such profound changes in neurological functioning during sleep, as the study shows, have immense effects on mood, memory and cognition. The natural question to ask once you identify this link between sleep and cognitive function is how many hours of sleep per day do you actually need for optimal cognitive performance? A new study published in JAMA Network Open claims to have the answer.
Sleep duration and cognitive decline
The study begins by stating that the proportion of older people in the global population is steadily rising, so much so that estimates say that by 2050, one-fifth of the global population will be above 60 years old. This inevitably means that the number of adults with cognitive impairment and dementia will also rise.
Some previous studies had observed sleep duration to be statistically associated with cognitive decline and dementia risks but these focussed on sleep deprivation alone and had very small sample sizes. On the other hand, the effect of excess sleep on cognitive function has not been studied much and whether it impairs cognitive abilities at all is unknown conclusively. This is the reason why the researchers behind this study undertook a large cohort study to explore the association between sleep duration and the trajectory of cognitive decline.
The researchers collected data from two randomly controlled cohorts of participants comprising 28,756 individuals; some living in England who were 50 years or older and others living in China who were 45 years or older. The participants were pooled from the English Longitudinal Study of Ageing (ELSA) and the China Health and Retirement Longitudinal Study (CHARLS) roughly between 2008 and 2017. Around 20,065 participants (9,254 from ELSA and 10,811 from CHARLS) were finally studied. The sleep duration was self-reported by the participants, while the cognitive assessment was done through questionnaires to examine memory, executive function and orientation.
What’s the best sleep duration for optimal cognitive function?
The researchers found that a U-shaped association existed between sleep duration and global cognitive decline. A U-shaped or curvilinear relationship signifies one where both extremes of the curve - indicating two extreme variables that affect the relationship - have the same or similar results. In this case, the researchers observed that participants who slept four hours or less every night experienced a faster cognitive decline. The researchers also observed a similar speed of cognitive decline among participants who slept 10 hours or more every night. This clearly indicates that both sleeping too much and sleeping too little was statistically and significantly associated with faster cognitive decline.
The fact that these findings were consistent in both ELSA and CHARLS participants shows that this U-shaped association between sleep and cognitive decline remained the same despite substantial ethnic and cultural differences between the participants, thus adding further to the validity of the findings. The study also found that an average sleep duration of seven hours per night seemed to be located at the top of the bell curve, therefore indicating the ideal number of sleep hours per night.
For more information, read our article on Sleep chart by age and gender.
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