The cognitive hazards of too little and too much sleep

Increasing evidence has shown that getting too little sleep increases the risk of developing dementia.  In one recent study those routinely getting 6 or less hours of sleep had a significantly greater chance of developing dementia than those getting 7 or more hours of sleep.  One suggestion has been that during sleep, the brain is cleansed of amyloid and other toxins by fluid moving through the so-called glymphatic circulation within the brain, a network of perivascular vessels likened to the lymphatic system in other parts of the body. 

An interesting paper published today online in JAMA Neurology suggests that the relationship between sleep and cognitive impairment may be more complicated. Over 4000 subjects without clinical dementia aged 65 to 85 years were screened with amyloid PET scans and cognitive tests. Those reporting sleeping less than 7 hours per night did significantly worse on memory tests than those getting 8 hours. They also had more amyloid in their brains. Surprisingly (at least to me), those getting excessive sleep, more than 9 hours nightly, also had more amyloid in their brains (see graph above), and they did worse on tests of executive function such as making plans. The under- and over-sleepers both had signs of poor health such as an increased BMI. For me this makes the sleep question more complicated: are they sleeping less or more because of other health issues, issues that might independently increase risk for amyloid deposition and Alzheimer’s disease, or is the 8-hour target really ideal for maximizing brain health?  More studies will be needed to sort this out, but in the meantime, as my mother tried to convince me, use moderation in all things.