Sleep plays an important role in cognitive and emotional processes.
Poor sleep – whether too long or too short – may be linked to cognitive impairment or dementia. In recent years, more and more studies have revealed that there is a two-way relationship between sleep and dementia: Sleep disorders are not necessarily caused by dementia, but rather sleep disorders may be one of the underlying factors in the development of dementia.
What is the effect of a lack of sleep?
Sleep disturbances are common and known to be part of the symptoms of dementia.
Lack of sleep may increase the risk of Alzheimer's dementia. This is because sleep deprivation significantly increases the presence of a substance known as amyloid-beta protein in the cerebrospinal fluid (CSF). Toxic accumulations of this protein are one of the biological hallmarks of Alzheimer's disease.
A 2017 study found that during a very specific phase of sleep, this substance is “cleaned” from the nervous system. This phase of sleep is called “deep, non-dream sleep,” and it is possible to identify exactly when this happens by monitoring brain waves with an EEG device. An interesting finding from the study is that the “cleaning” occurs specifically for this protein, and not for other proteins in the spinal fluid. It is important that this phase of sleep occurs every night.
Lack of sleep may increase the risk of Alzheimer's dementia.
What is the effect of sleeping too much?
Several studies have found a link between sleep duration and an increased incidence of dementia.
A 2017 study compared people who slept more than 9 hours per night and found that prolonged sleep was a significant predictor of dementia. People who reported sleeping more than 9 hours (compared to 6-9 hours) had smaller brain volume (as measured by MRI) and more difficulty performing cognitive tasks.
The people who slept more than 9 had not done so in the past, and during the 13-year longitudinal study, it became clear that there was a significant increase in the number of hours of sleep per night.
These findings revealed that increased sleep duration is an early sign of a degenerative process in the brain.
Another study, conducted in Japan in 2018 on a very large sample, also found that a significant increase in sleep duration over 12 years was associated with a significant increase in the risk of dementia.
The reason why longer sleep duration affects the brain in this way is unknown. One possible explanation is that longer sleep duration actually reflects poor sleep quality. Previous studies have found that poor sleep quality may be a risk factor for dementia.
Another possible explanation is related to Obstructive Sleep Apnea (OSA). This is a sleep disorder that affects 5% of the population, in which the person experiences periodic pauses in breathing during sleep at night.
recommendation:
At least 5 hours and up to 9 hours at night This is the most current recommendation (August 2020).
Daytime sleep is not included – this sleep is usually short and does not include the important phase of "cleaning" the spinal and brain fluid.
Sources:
Irwin, M & Vitellio, M. (2019). Implications of sleep disturbance and inflammation for Alzheimer's disease dementia. Lancet Neurology. Published online Jan. 17, 2019.
Livingston, G. et al. (2020). Dementia prevention, intervention, and care: 2020 report of the Lancet Commission. Thelancet.com, 396.
Lu, Y., Sugawara, Y., Zhang, S., Tomata, Y. & Tsuji, I. (2018). Changes in sleep duration and the risk of incident dementia in the elderly Japanese: the Ohsaki Cohort 2006 Study. SLEEPJ, 1–8.
Westwood, A., Beiser, A., Himali, N., Auerbach, S. & Seshadri, S. (2017). Prolonged sleep duration as a marker of early neurodegeneration predicting incident dementia. Neurology, 88, 1172–1179.