Reading Matthew Walker’s ‘Why we sleep’ was a key experience, opening up information about new understandings of sleep and its importance to health and development in human beings and other creatures. Why We Sleep - Wikipedia
Walker recommends 7-8 hours sleep every night to preserve good physical and mental health and to aid recovery in the face of illness or trauma.
His research and that of others have found that broken sleep predicts the emergence of Alzheimer’s disease. Good nights’ sleep is associated with preserved cognition: How we sleep today may forecast when Alzheimer’s disease begins | Berkeley News
Reports from recent research tell us now about the relationship between daytime naps and Alzheimer’s disease Long naps may be early sign of Alzheimer’s disease, study shows | Alzheimer's | The Guardian
This is impressive work led by Kun Hu of the Medical Biodynamics Programme – Brigham’s Division of Sleep and Circadian Disorders. It uses data from people enrolled with the Rush Memory and Aging Project (MAP) Overview and Findings from the Rush Memory and Aging Project - PMC (nih.gov)
This was setup from 1997. From 2005 recruits were fitted with wrist monitors which identify periods of activity and inactivity (Actigraph). Inactivity is interpreted as a proxy for sleep.
1401 older people (1065f, 336m) were included in this study, average age 81.4 years. Their cognition was assessed using a battery of 21 tests each year. Where appropriate a clinical diagnosis of Alzheimer’s disease was established.
At baseline 812 had no cognitive impairment and only 4.4% had a diagnosis of dementia. During the study 384 developed MCI and 101 developed Alzheimer’s disease.
People spending more time in daytime naps were more likely to develop MCI or dementia.
The frequency and duration of naps doubled with a diagnosis of MCI and trebled with a diagnosis of dementia.
The interpretation is that extra daytime napping is an early sign of cognitive deterioration and/or the development of Alzheimer’s disease. Napping does increase with age and not every nap predicts the coming of dementia, but other researchers have found that there is loss of biochemical activity in neurones in brain centres which are involved in the sleep-wakefulness activity of the brain: the locus coeruleus, the lateral hypothalamus and the tubero-mamillary nucleus. So extra naps may be evidence of neuro-degeneration in these key nuclei.
We do not know if approaches to reduce napping will prevent or slow deterioration in cognition. For the present it will be wise to follow Matthew Walker’s advice to attend to sleep hygiene, to obtain a good night’s sleep for every night.