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Sleep duration and the risk of dementia

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As dementia results from changes in the brain, it is unsurprising that people with dementia often have disrupted sleep patterns. Changes in sleep pattern may occur before a person has enough other symptoms to acquire a diagnosis of dementia. Previous research has shown that disrupted sleep patterns may occur several years before dementia becomes manifest.

A study that has just been published in the high profile journal Nature Communications by Sabia and colleagues explores the association of sleep duration in middle and old age with the incidence of dementia. This important study contributes to the existing literature in several ways. The authors have used data from the UK Whitehall II cohort – civil servants who were first interviewed between age 35 and 55 and then followed up for about 25 years. There were nearly 8000 participants with data on their self-reported sleep, of whom just over 500 had developed dementia by the end of the period being studied. Men and women with shorter sleep duration (6 hours a night or less) at the age of 50 or at the age of 60 had a higher rate of developing dementia than those who slept 7 hours per night. The findings for people sleeping 8 hours plus were less clear. Given that the mean age of dementia diagnosis was 77, then these changes occurred many years before the condition was diagnosed.

What this study adds is the large sample size, the full assessments and the long period of follow-up. The study has made repeated measures of sleep duration over the years. Although most of the sleep data were based on self-report, which can be unreliable, a small subset of participants did have objective sleep measures with an accelerometer and the findings were consistent with the rest of the observations. The study team were able to use NHS data to determine whether participants had eventually been diagnosed with dementia. While NHS data have their limitations, this method will correctly pick up the great majority of cases. Finally, the research team undertook several sensitivity analyses, that is analyses to discount other possible hypotheses;  for example, they were able to establish that the effects of sleep on the future risk of dementia were not due to mental illness, such as depression or anxiety, as these also play havoc with sleep.

What’s the message for us all? Evidence of sleep disturbance can occur a long time before the onset of other clinical evidence of dementia. However, this study cannot establish cause and effect. Maybe it is simply a very early sign of the dementia that is to come, but it’s also quite likely that poor sleep is not good for the brain and leaves it vulnerable to neurodegenerative conditions like Alzheimer’s disease.

Turning off mobile phones and avoiding caffeine before bed are good habits to have as we already know the importance of good sleep on health more generally. However, however we would need further studies to know if longer sleep in itself could reduce the risk of dementia later in life.

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