Enough regular sleep helps to prevent dementia

For most people, getting too little sleep one night is associated with mental fatigue the next day. The hours pass slowly and densely, marked by a sense of unreality. The brain reacts rather sluggishly. We think worse, we forget things, we find it hard to stay focused. If the lack of sleeping time is significant, we are overcome by a certain confusion, as if the parts of the day do not quite fit together.

When this occasional lack of sleep becomes systematic, prolonged over time, a sort of cumulative effect develops. Neuroscience has sufficiently demonstrated, based on overwhelming evidence, that getting too little sleep as a norm — over years or decades — increases the risk of cognitive impairment in later life.

There are several studies that back up the mental harm caused by insufficient sleep. One study published in 2021 by Nature concluded that sleeping six hours or less — the sleep duration of nearly 8,000 participants at ages 50, 60 and 70 was measured — increases the likelihood of Alzheimer’s and other types of dementia by 30%. Another analysis published that same year, by researchers at Harvard Medical School, yielded even more compelling results: those who sleep less than five hours are twice as likely to develop dementia as those who get an average of seven hours’ sleep.

However, two recent research studies suggest that it is not only duration that counts. Both coincide in indicating — albeit from different approaches — that regularity in sleep patterns could also significantly influence our cognition. It does not seem advisable to alternate between highly variable sleep times. It is also not entirely harmless to frequently change the time period in which we sleep.

Jeffrey Iliff, a researcher in sleep and health, spearheaded one of these two new contributions to a burgeoning field of analysis. By cross-referencing data from the Seattle Longitudinal Study, which has been collecting psychosocial information on thousands of individuals since 1956, Iliff and his team sought to learn more about the link between stability in sleep quantity (measured over 20 years) and the subsequent onset of dementia.

Via a video conference call, Iliff summarizes their main finding: “Those who progressively decrease their hours of sleep are not at the greatest risk of cognitive impairment, but rather those who vary the most in the number of hours slept.” People who get insufficient sleep during one season and who, in other seasons, toss and turn all night. And then they return to sleep very little. And, after months or years, they sleep too much again. And so on.

Iliff admits that, for the time being, we can only speculate about the causes of this close correlation between poor sleep and cognitive impairment. “It is possible that variability is, on its own, a factor to be taken into account. But it is also plausible that other factors associated with an increased risk of dementia (chronic disease, apnea, depression, etc.) cause this variability,” he explains.

The second study on sleep patterns and dementia, carried out by Australian and Canadian researchers, focuses on the regularity of schedules. Going to bed with no set order (one day at 10 p.m.; another, say, at 3 a.m.), and repeating this disorder as a rule, leads to a significant increase in the risk of suffering from Alzheimer’s or other neurodegenerative diseases in the future, the study suggests. One of the authors, Matthew Pase, a researcher at Monash University, ventures into the domain of conjecture. “Cardiovascular diseases are more frequent among people that have an irregular sleep pattern. These pathologies cause the blood supply to the brain to function more poorly, which may help to explain some of the long-term cognitive impairment,” he says.

In the relationship between sleep and cognition, where multiple elements converge in a complex equation, some evidence confirms what has already been foreseen by common sense. On the contrary, others seem counter-intuitive. A number of studies have concluded, for example, that sleeping too much (over 9-10 hours) also increases the likelihood of experiencing a gradual loss of cognitive abilities. In 2017, a meta-analysis identified this finding in 10 publications. Another multi-study study published in 2019 showed a 77% increase in the risk of dementia among sleepers compared to those who remain in the optimal range, estimated at about seven to eight hours.

In the opinion of Mercè Mayos, member of the Spanish Federation of Sleep Medicine Societies, a central concept would resolve this apparent paradox: comorbidity. In other words, the presence of two or more pathologies whose symptoms and mechanisms are sometimes difficult to observe individually. “Of course, it seems nonsensical that too much sleep is bad at the cognitive level. The main hypothesis is that there must be confounding factors: depression or other comorbidities that lead these people to sleep more.”

The study on sleep schedules in which Pase participated also involves a dose of oddity. If sleeping and waking without a more or less fixed pattern could be compromising our future cognitive capacity, the risk of dementia also increases when rest is subject to scrupulously strict schedules. For instance, someone who sleeps from 11 p.m. to 7 a.m. with dogged perseverance, with hardly any exceptions, at the stroke of a clock. Pase suggests, as a feasible motive and suggestive line of research, another possible angle, in this case a relational one: “Perhaps people who are very strict with their sleep schedules have a very limited social life, which is not exactly conducive to sound cognitive health.”

On fertile ground for exploration, a phenomenon discovered only a decade ago helps to understand why poor sleep (in quantity and quality) compromises our cognition. “We now know that one of the main functions of sleep is to cleanse the neurotoxicity that we generate during the day. If we sleep badly, substances accumulate that contribute to neurodegeneration,” stresses Javier Albares, director of the sleep unit at Centro Médico Teknon (Barcelona).

Knowledge of the existence of a glymphatic system — a term coined by the Danish neuroscientist Maiken Nedergaard, who in 2012 conceptualized this mechanism — has become a beacon that guides the growing literature on sleep and cognitive impairment. “It acts as a network of spleens that eliminates residues from the central nervous system, especially fibrous protein closely related to Alzheimer’s disease, frontotemporal dementia or Parkinson’s disease,” summarizes Mayos. Albares adds that “these brain-cleansing processes are activated especially during non-REM stage 3, when deep slow-wave sleep occurs.”

Mayos recommends “making sleep a pillar of health on a par with nutrition and physical activity.” And she regrets that this field is still like “the Cinderella of medicine.” Scarce consideration that is reflected in our habits and collective imaginary: “Socially, it is trivialized to sleep little, and even rewarded by giving it a positive connotation for the sake of supposedly greater productivity.” Matthew Pase, who spent years working in the U.S., testifies to how “well regarded” it is to get up very early over there.

Things are gradually starting to change. Mayos provides as proof an influential article that appeared last October in The Lancet appealing to put sleep “on public health agendas” around the world. Pase concludes: “We know more and more about its importance for good health over the course of our lives. It is time to pass the message on to the population.”

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