Ah, sleep! When you get the right amount of it, you feel great. But if you don’t sleep too well for a night or two, or worse, lose a night of sleep entirely, you can barely function. You, like all other human beings, have had a lifetime of personal experience with sleep, and have directly experienced how much of an impact it can have on how you function, or don’t!

So it should come as no big surprise that sleep plays an absolutely fundamental role in the optimal functioning of your brain, and is critical for cognitive and emotional processing, as well as memory and psychological health. Sleep also protects your brain by actually clearing waste products from the neural tissue while you sleep.

Researchers have long known that changes in one of the characteristics of sleep, namely how many hours of sleep you get each night, known as sleep duration, is linked to several conditions, including cardiovascular and cerebrovascular (brain blood vessel) disease as well as dementia.

A Non-Linear Relationship

But here’s where things get interesting. The relationship between sleep duration and the onset of these diseases is not exactly straightforward. It turns out that both too little sleep (6.5 hours or less per night) or too much sleep (more than 9 hours) are both linked to an increased risk. Definitely not a case of “more is better!”

Past research has shown that for each hour of reduced sleep duration there was an increase of 0.59 percent in the volume of the ventricles in study participants who were 55 years of age or older. The brain’s ventricles are a communicating network of cavities located deep within the brain tissue and filled with cerebrospinal fluid. The expansion of these ventricles, as shown on MRI, has long been associated with the progression of Alzheimer's disease.

Shorter sleep duration is also related to changes in the structure of the brain’s white matter, the tissue in the brain that is composed of nerve fibres. This explains why so many patients with dementia have a decline in their motor function, and subsequently develop difficulty with walking, picking up objects and even feeding or dressing themselves. It seems that previous research studies looking at sleep duration and brain structure were all focused on linear relationships, and not the clearly non-linear relationship described above, where either too little or too much sleep were both detrimental.

A New Study

But now, a new study by researchers from China’s Fudan University and the UK’s University of Cambridge, published in April 2022 in the prestigious journal Nature Aging, aims to correct this by directly examining this non-linear relationship. The scientists took data from a database known as the UK Biobank. This database is a very large-scale repository of genetic and health information from UK participants, and is widely used as a research resource. The information contained in the database includes cognitive assessments, mental health questionnaires, brain imaging studies and in-depth genetic information.

The researchers looked at data from some 500,000 adults aged 38 to 73 years. The team’s educated guess (in scientific terms, their hypothesis) going into the study was that this same non-linear relationship between sleep duration and the development of dementia would also be true of the relationship between sleep duration and mental health, as well as between sleep duration, cognition, and the structure of the brain itself.

The Results

The research showed a definite association between too little sleep as well as too much sleep, on poor performance on cognitive tasks, including memory and reaction time. Sleep duration also showed this same two-sided relationship with mental health symptoms, including symptoms of depression, anxiety, and even self-harm behaviours. Even changes in brain structure, as determined by imaging data, showed this same nonlinear association, particularly parts of the brain affected in dementia.

This study also looked at these non-linear patterns as they were reflected by different age groups. Their results showed that as the participants got older, there was a decrease both in brain volume and cognitive function, with the most significant relationship showing up for participants between the ages of 44 to 59 years. Interestingly the results were a bit different when it came to mental health. The study clearly showed that the association between the participant’s sleep duration and their mental health and cognition gradually decreased as the participants approached 65 and beyond.

The researchers also further strengthened their findings by following up with those people who had non-optimal sleep duration at the beginning of the study. Follow up revealed that over time there was a decrease in these participant’s cognitive function and an increase in psychiatric symptoms.

What This Study Means For Your Cognitive Health

So, all this data may be interesting, but what does it mean in practical terms for your health? Most importantly, is there an optimal number of hours of sleep that you should get to preserve your cognitive function and your emotional health?

The big takeaway from this study is that the regulation of your sleep, in terms of its duration, is vital for optimal brain functioning as well as your mental health and wellbeing. Sleep also seems to be in a pathway that also includes genetics and other brain mechanisms.

The Goldilocks Rule

The study identified a consistent sleep pattern of approximately seven hours as being the best for optimal long term health. To paraphrase Goldilocks, “not too little and not too much, but just right.”

The researchers further underscore that for those people who have jobs that require them to do shift work and/or travel, this recommendation is particularly important. Keep in mind that too little sleep as well as too much sleep has been shown to be detrimental.

This does not mean you have to sleep exactly seven hours every night. You may do fine on a little less or a little more sleep, but you certainly should not be sleeping several hours less or more than seven.

The other point the study made is around sleep consistency. You don’t want the number of hours you sleep each night to be jumping all over the place every week, such as five hours one night then six the next and eight for the next couple of nights and so forth. Try your best, within reason to keep the number of hours you sleep each night near a consistent number, ideally seven.

Many individuals, as they grow older, experience a change in their sleep patterns. These changes include difficulty falling asleep as well as staying asleep, and a decrease in both the quantity and quality of the sleep they do get. Optimization of sleep is a whole topic in itself, but the most important tips are to stick to a relaxing evening routine, avoid bright light exposure in the evenings, especially the blue light from electronic screens and LED light bulbs, avoid caffeine intake at least six hours before bed, and limit fluid intake after 8pm.

The other very effective thing you can do in the morning, as soon as you get up and before you are exposed to artificial light, is to get outside and get some sunlight on your face for at least five minutes. This natural light exposure will set your master circadian clock for the day and will set you up for a good sleep that night.

Of course, fine tuning your sleep is only one part in the whole journey toward optimizing your health. You can do this with a combination of approaches, not only by getting consistent quality sleep, but with exercise, stress control, nutrition and targeted supplementation.

References:

1. Sean M. Nestor, Raul Rupsingh, Michael Borrie, Matthew Smith, Vittorio Accomazzi, Jennie L. Wells, Jennifer Fogarty, Robert Bartha, the Alzheimer's Disease Neuroimaging Initiative, Ventricular enlargement as a possible measure of Alzheimer's disease progression validated using the Alzheimer's disease neuroimaging initiative database, Brain, Volume 131, Issue 9, September 2008, Pages 2443–2454, https://doi.org/10.1093/brain/awn146.

2. Li, Y, Sahakian, BJ, et al. The brain structure and genetic mechanisms underlying the nonlinear association between sleep duration, cognition, and mental health. Nature Aging; 28 Apr 2022 | DOI:10.1038/s43587-022-00210-2

3. Westwood, A. J. et al. Prolonged sleep duration as a marker of early neurodegeneration predicting incident dementia. Neurology 88, 1172–1179 (2017).

4. Sabia, S. et al. Association of sleep duration in middle and old age with incidence of dementia. Nat. Commun. 12, 2289 (2021).

5. Ma, Y. J. et al. Association between sleep duration and cognitive decline. JAMA Netw. Open 3, e2013573 (2020).

6. Lo, J. C., Loh, K. K., Zheng, H., Sim, S. K. Y. & Chee, M. W. L. Sleep duration and age-related changes in brain structure and cognitive performance. Sleep 37, 1171–1178 (2014).

7. Xu, W. et al. Sleep characteristics and cerebrospinal fluid biomarkers of Alzheimer’s disease pathology in cognitively intact older adults: the CABLE study. Alzheimers Dement. 16, 1146–1152 (2020).

8. Liang, Y., Qu, L. B. & Liu, H. Non-linear associations between sleep duration and the risks of mild cognitive impairment/dementia and cognitive decline: a dose–response meta-analysis of observational studies. Aging Clin. Exp. Res. 31, 309–320 (2019).