My Mom told me that I was supposed to get eight hours of sleep at night. Didn't every Mom say that? And I think we also heard that in health class and from the sleep experts. But, older people have a harder time sleeping, and I've heard people say that older people don't really need as much sleep as younger people. That's why they sleep less. Is that true? And sometimes people ask me, is it bad when older people get too much sleep? (My Mom reads all of these posts, so I may hear from her about this.)
We've mentioned before Dr. Matthew Walker and his widely respected book Why We Sleep. Dr. Walker contends that everyone needs 7-8 hours a night, although he strongly favors the 8-hour end of that spectrum. We've also written before that atrophy of the different parts of the brain makes it harder for us to sleep and especially to stay asleep once we've begun. Dr. Walker says we still need to do what it takes to get our seven or eight hours in. So just because we sleep less doesn't mean we need less -- we still need the same amount. And failing to sleep straight through seven or eight hours in one sleep cycle encourages the buildup of beta amyloid in the brain. That's part of the Alzheimer's disease process.
You may have seen reports of a new study on hours of sleep in middle aged and older people. It was published a few days ago in the journal Nature Ageing. It's entitled "The brain structure and genetic mechanisms underlying the nonlinear association between sleep duration, cognition and mental health." If you don't want to pay $32 to download the journal article, you can read more about it here in Science Daily.
In popular media, the study is important for telling us that the optimal sleep period for middle aged and older people is seven hours, and getting too much sleep is bad for your memory and cognition. But didn't we already know that? We wrote about similar findings in another study last year. (The new study does add that disrupting your nightly sleep schedule and accumulated hours of sleep can increase problems with cognition and mental health.)
But there's actually more.
The title refers to a "non-linear association." What's that about? Here is a set of graphs from their data. They relate hours of sleep to performance on a set of cognitive tests. I recognize these tests as those that I get periodically from my neurology care team in Seattle. You'll note that all of the curves find their optimum point around seven hours, although some peak more at seven and a half or eight hours.
Particularly interesting was the association between optimal sleep and brain structure. Variation in the thickness of layers of cortex and the size of structures were all associated with variation in hours of sleep. The researchers specifically found variations in the volumes of the precentral cortex, the lateral orbitofrontal cortex and the hippocampus were all associated with variations in sleep. They referred to "damage" to the hippocampus in people who don't get the optimal amount of sleep. Given the role of the hippocampus in memory processes and in Alzheimer’s disease, the nonlinear association between sleep duration and this brain region is of particular importance to us.
The article is telling many people something about optimal sleep duration that they probably hadn't heard yet. That's because they don't read this blog. But it seems the article is attracting more interest in scientific circles because it provides new insights on why seven to eight hours would be optimal. It looked at many parts of the brain involved in memory and cognition, and it addressed how these parts are affected by varying sleep habits.
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