Short sleep a day keeps your memories far away

Welcome to this edition's?Sleepletter?where we offer you easy-to-read insights from the latest research papers from the field of?sleep neurobiology?and clinical?sleep medicine. We hope you enjoy the content!

Chronic short sleep and its association with dementia

We’ve all experienced short-term complete or partial lack of sleep in our lives. The next day we don’t feel great. In fact, we may have emotional swings, lack of concentration, poorer memory and other cognitive issues. Not cool, right? Just try imagining the detrimental effects to your brain when sleeping less than 5 or 6 hours almost every day for the next 5 or 10 or 20 years. This is where the association of dementia and poor sleep comes from. Many people with Alzheimer’s disease, the most common dementia, as well as other forms of dementia experience sleep problems. This is likely due to neurons dying off in the brainstem and brain’s hypothalamus, both of which are important in controlling the sleep-wake cycle, especially a group of neurons in the hypothalamic suprachiasmatic nucleus called the “master clock” which regulates the 24-hour circadian “sleep-wake” rhythm. So, we know that dementia can cause sleep problems, but can poor sleep lead to dementia too? Some studies indicate so, but most of them followed patients for less than 10 years. While it’s still a long time, most dementias are characterized by changes over 20+ years. So, a much longer study is needed to give a quality assessment. Fortunately, this is just what we got in a recent Nature Communications paper.

An international group of scientists led by the French Inserm Institute and University College London used data from almost 8000 participants to examine the association between sleep duration and the incidence of dementia over a 25 year period. Sleep duration was assessed using sleep questionnaires (subjective sleep examination) and accelerometers (objective sleep examination), depending on the subgroup. All participants entered the study at the age of roughly 50. At the beginning of the study, almost 40% reported sleeping less than 6 hours (short sleepers), 45% slept 7 hours (normal sleepers) and 15% slept longer than 8 hours (long sleepers).?

Overall, 6.5% of participants developed dementia. The lowest levels of dementia incidence were observed among normal sleepers, irrespective of the age during the study (50, 60 or 70 years). On the other hand, short sleepers at the age of 50 were associated with a 22% increase in risk of dementia, and 37% increase in risk at the age of 60. However, there was no significant association between short sleep and dementia at the age of 70 or long sleep with dementia at any age. Data from accelerometers also confirmed the association of short sleep with dementia, but with an even higher increase in risk, about 90% at 6 hours of sleep, and higher still as sleep duration shortened. Since mental disorders per se can increase the risk of dementia, a subgroup of short sleepers without any mental disorders was examined and the researchers found similar results, suggesting that short sleep, independently of mental disorders increases the risk of dementia.?

But what happens if a person sleeps poorly for a short period of time and then becomes a normal sleeper? Interestingly, over a period of 20 years (between the ages of 50 and 70), only persistent short sleepers were associated with a significant increase in risk of dementia (32%), whereas change from short sleep to normal, or normal to short was not. This may suggest that only long-term short sleep (15-20 years) increases the dementia risk, whereas periods of short sleep, followed by normal sleep do not. Of course, this intriguing hypothesis needs further investigation.

Nevertheless, this study shows and highlights the importance of normal sleep duration on brain health over a course of 25 years. Therefore, even if you might think that you can function with less than optimal sleep duration during your adult life, your brain might give you a completely different answer.

Study: https://www.nature.com/articles/s41467-021-22354-2?

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About the author

Alen Juginovi? is a medical doctor and?postdoctoral researcher in the Department of Neurobiology at Harvard Medical School in Boston, MA, USA studying the effect of poor sleep quality on health. He is also organizing international award-winning projects such as Nobel Laureate conferences, international congresses, concerts and other, as well as participating in many events as a speaker. Feel free to contact him via LinkedIn for any inquiries.