Sleep is a fundamental moment for our body, in which we recover energy, dispose of toxins, rebalance our metabolism, consolidate memories and experiences of the day. Everyone probably knows that, for this reason, sleeping too little is very harmful to your health. Less known, perhaps, is that too much is also a bad thing: too many hours of night sleep can be a sign of some problem. How much sleep should we sleep, therefore, to age healthily and avoid serious disorders such as Alzheimer’s and other forms of dementia?
The answer comes from a new study from York University in Toronto, which analyzed data collected from 69 previous studies to evaluate the association between sleep duration, physical activity, time spent sitting and the onset of dementia. The research, published in Plos One, examined a total sample of around four and a half million people, arriving at a definitive answer: the optimal amount of nightly sleep is between seven and eight hours, an interval associated with a lower incidence of cognitive disorders in old age.
Delay cognitive decline
Medicine has long been trying to identify modifiable lifestyle factors that can prevent or delay the onset of dementia, a disorder with an ever-increasing prevalence, for which there is not yet a definitive cure. It is estimated that about half of dementia cases could be avoided by acting on behavioral and environmental risk factors. To shed light on this front, the Canadian team carried out a systematic review of the available scientific literature, combining data from sixty-nine independent studies that had followed participants initially without signs of dementia.
The strength of this analysis lies in the vastness of the sample examined, which includes approximately four million people, and in the decision to also include relatively young age groups, starting from the age of thirty-five, thus anticipating the observation compared to most traditional epidemiological investigations.
The results
The work focused on three modifiable behavioral variables: the duration of night sleep, the amount of physical activity practiced weekly and the time spent in a sitting position during the day. The analysis found that sleeping less than 7 hours per night is associated with an 18 percent increase in the risk of dementia. On the other hand, prolonged rest beyond 8 hours a day is correlated with a 28 percent increase in risk, confirming that sleeping too much can be just as harmful to neurological health as insufficient rest.
The range between seven and eight hours of sleep therefore represents the ideal balance point that emerged from the research. The analysis also highlighted the weight of other related behaviors, such as physical activity of less than one hundred and fifty minutes per week and prolonged sedentary behavior exceeding eight hours per day, both factors linked to greater cognitive vulnerability.
The biological mechanisms and the limits of the research
While we cannot establish a direct cause and effect relationship due to the observational nature of the included studies, the research suggests probable biological mechanisms through which these behaviors may act on the brain. According to the authors, a healthy movement and rest profile promotes vascular, neurotrophic and anti-inflammatory effects that help preserve brain health. Regular physical activity, absence of excessive sedentary lifestyle and correct duration of sleep combine to keep cerebral blood flow active, facilitate the removal of waste products from neurons and protect related organs such as the cardiovascular system.
However, the researchers themselves specify the need to maintain caution when interpreting the data, since distinguishing between causes and clinical consequences remains complex: in many cases, for example, excessive sleep could represent an early sign of cognitive degeneration that has already begun, and not a modifiable risk factor.