What effect does sleep have on dementia?

It is true that one can have trouble sleeping even when healthy. But, for some people, disturbed sleep could be an early sign of dementia. Symptoms like this may show up years before getting a proper diagnosis, and although experts are not sure which comes first, there is evidence that bad sleep and dementia are interrelated.

Interrupted sleep matters

Good sleep is important at any age. Contrary to the resting physical state, the brain remains very active during sleep, carrying out many important functions. Sleep is essential to every process in the body, whether it is affecting our overall physical and mental functioning, fighting diseases, developing immunity, or reducing chronic disease risk. It is recommended that adults (including older people) get seven to nine hours of sleep every day. While waking up once or twice briefly during the night is normal, interrupted or fragmented sleep for a prolonged period of time can have serious consequences.

Sleep apnoea, or sleep-disordered breathing, is one such common sleep disorder, characterised by repeated awakenings and pauses in breathing during the night. People with this disorder often snore and gasp during sleep, and it has been associated with hypertension (high blood pressure), cardiovascular disease, and diabetes. Now a new study suggests it may also be associated with cognitive decline and dementia.

The study, published in the Journal of the American Medical Association, is not the first to find a connection between sleep apnoea and dementia, but it is the first to use an objective measure of sleep-disordered breathing – actually counting the number of disordered breathing events during a night of sleep using electroencephalography (EEG) scans and other equipment. It is also the first to study the matter prospectively, finding the disordered sleeping pattern in healthy people without dementia and then checking, years later, to see if they were more likely than regular sleepers to deteriorate cognitively.

Evidence of causation?

Sleep plays an important role in the consolidation of long-term memories, so the scientists at first suspected that the frequent interruptions of sleep, and getting less sleep, might have caused the decline in cognitive function. But the main association with cognitive damage, found the scientists, stemmed from hypoxia – reduced oxygen to the brain caused by the breathing disruptions. The underlying theory is that brain cells are extremely sensitive to oxygen deprivation and can potentially begin to die within five minutes if oxygen supply gets cut off. During apnoea events, the body is denied oxygen. Normal blood oxygenation is at 90%- 95% – an apnoea event can drive that level into the 80s or even 70s. These levels of oxygenation are damaging and dangerous, so much so that it can lead to cognitive impairment.

The authors also noted a key limitation: the sleep data was only gathered for one night, so possible variability in sleep disturbances over time was not captured. But the objective measures of sleep-disordered breathing did give them an indication that sleep apnoea is associated with cognitive decline. Several other studies also demonstrate this association between hypoxia and dementia, but no study so far has been fully able to prove a causative effect. There have been many other studies researching the relationship between sleep and dementia. One study found that the temporal lobes, which are vital for memory, were reduced in thickness in those with sleep apnoea, a change that is also seen in people with dementia.

Another study did a brain tissue autopsy in patients suffering from sleep apnea and suggested that the hippocampus was reduced in volume in people with sleep apnoea. Hippocampal atrophy is a feature of Alzheimer’s disease. This study further showed that two toxins, tau and beta-amyloid, which are thought to be responsible for many of the symptoms of dementia, build up in the brains of people with sleep apnoea, probably because of a lack of oxygen in the blood. DPUK has written about the proteins involved in dementia in this blog.  

Dr Liz Coulthard, an Associate Professor in Dementia Neurology at the University of Bristol, leads the ReMemBr research group comprising neurologists, psychologists and others. In her recent talk at a DPUK-led public event, she spoke about the process of sleeping and how it may be important in preventing the onset of dementia. Sleep is a very active process – one that is complementary to being awake. During the slow-wave stage of sleep, natural brain oscillations may be responsible for the filtering of toxins from the brain. One such toxin is amyloid, a protein which accumulates during dementia. By improving our sleep, it is possible that we could reduce the amount of amyloid and therefore reduce our risk of dementia.

Implications for treatment

Previous research has identified a link between dementia and common sleep disorders, including sleep apnea. However, further studies using larger sample sizes will be necessary to validate the findings. If there is a relationship between sleep and dementia, it is not entirely clear why it exists.  Studies have documented a number of sleep changes that occur in healthy ageing adults, including waketime and bedtime shifting, taking longer to fall asleep once in bed, experiencing fragmented sleep, sleeping fewer hours per night, and spending less time in slow-wave and rapid eye movement (REM) sleep. REM sleep behaviour disorder can cause individuals to act out their dreams – sometimes in dangerous ways. It is most often found in individuals with Lewy body dementia and is sometimes the first symptom that arises with this type of dementia. ‘Sundowning’ is another phenomenon in which individuals with dementia experience increased agitation later in the day and in the evening. Sundowning can contribute to insomnia and other sleep problems when these behaviours continue into the night. People with dementia may also talk, yell, or cry out at night if they cannot sleep. Although these changes parallel some of the sleep challenges seen in people without dementia. However, the fact is that sleep pattern changes in dementia patients can be a lot more dramatic and disruptive.

Sleep could therefore have an effect on dementia either as a cause or as a symptom, and scientists are still untangling the complexities of dementia due to lack of sleep and fragmented sleep due to dementia. So far, the diseases causing dementia have no cure, so it is crucial to use all available strategies to treat dementia’s symptoms and boost prevention efforts – including by tackling sleep disruption. While the hunt for cause and effect continues, maintaining a healthy lifestyle and ensuring high-quality sleep can improve both short and long-term cognitive performance and potentially reduce the likelihood of age-related cognitive decline.