We have all experienced the impact of a short night’s sleep on our brain the day after. Similarly, a disrupted sleep pattern is one of the factors that affect cognitive functions as we age. But is it sleep duration or quality that matters? And how many hours of sleep does one need to function optimally after 50?
The World Health Organization’s study on global ageing and adult health (SAGE) used data from 6 different countries (China, Ghana, India, Russian Federation, South Africa, and Mexico) to try to answer these questions. The study began in 2007 and involved 30,000 participants of at least 50 years of age.
Participants estimated their sleep quality and duration in the past two nights based on a five-point scale. Next, they were given standard cognition tests that included the immediate and delayed recall of a list of words and working memory tests.
The study concluded that six to nine hours of sleep per night seemed ideal for seniors’ cognition. Sleep quality was also a predictor of better cognitive functioning.
Indeed, individuals who reported sleeping more than nine hours per night had lower cognitive test scores, as did those who sleep less than six hours per night.
Sex differences were also observed. Men generally had higher sleep quality while women reported longer sleep durations (except in Russia and Mexico, where men reported sleeping longer than women).
This study supports results previously observed in populations in the United States, Western Europe and Japan.
It is interesting to see similar results in countries differing culturally, economically and environmentally. Of note, men and women in South Africa sleep the most of the 6 countries concerned in the present study, while those in India sleep the least.
In sum, the right amount of sleep (6 to 9h), if possible of good quality, is essential for a sharp brain. This could be explained by the fact that, while we sleep, several biological processes occur in the brain that are linked to memory consolidation.
It is not clear though why too much sleep can comprise cognition. Other factors may be at play.
Along with physical exercise and intellectual stimulation, improving sleep patterns is thus critical to maintain and/or improve one’s brain fitness.
References : Gildner, T.E. et al. (2014). Associations between Sleep Duration, Sleep Quality, and Cognitive Test Performance among Older Adults from Six Middle Income Countries: Results from the Study on Global Ageing and Adult Health (SAGE). Journal of Clinical Sleep Medicine, 10(6).