Monday, August 6, 2018

Sleep, Thinking, and Aging

I have written a few times about the influence of sleep on thinking.  High school students who stay up late perform more poorly in school the following day.  A lack of sleep may cause you to mix together different memories that did not occur together.  In young adults, sleep also affects the ability to learn new procedures. 
These benefits of sleep lead naturally to the speculation that sleep may help older adults avoid the cognitive declines that come along with aging.  One possibility is that older adults who suffer from sleep difficulties decline faster than those who don’t.  Another possibility is that regular sleep throughout life is associated with lower levels of problems.
A paper in the January, 2015 issue of Perspectives on Psychological Science by Michael Scullin and Donald Bliwise tried to sort out what is going on with sleep and aging.  The performed a massive meta-analysis.  A meta-analysis looks across the many published studies in an area of research in order to explore what really seems to be happening in an area.
There are many ways to study sleep and its effects on thought and aging.  Some studies use self-reports of sleep quality and measurements of cognitive performance.  Some of these self-report studies look at people of different ages.  Others are actually longitudinal.  They examine the relationship between the quality of sleep people get at one point in time and their performance later in their life.
Other studies use other measures of sleep.  Some studies use a device called an actigraph, which measures whether the person is moving.  (The Fitbit is a kind of actigraph.)  Long periods without movement are good (though not perfect) signals that a person is sleeping.  Still other studies measure physiological aspects of individuals like brain waves so that it is possible to tell both that people are asleep as well as the stage of sleep they are in.  Finally, there are experimental manipulations of sleep including sleep deprivation studies as well as studies in which people are randomly assigned to conditions in which they do or do not nap.
There are a lot of interesting findings in this paper, and it is worth giving it a read yourself for a more complete look at effects of sleep on thinking.  But here are a few highlights.
First, the relationship between sleep and improved thinking is strongest earlier in life and gets weaker later.  A good night’s sleep helps young adults to learn better the next day.  Sleep also helps young adults to consolidate (or solidify) memories from the day before more than it helps older adults.  Middle-aged adults show smaller effects of sleep on learning, and older adults show almost no relationship between sleep and learning at all. 
Sleep deprivation studies tell the same story.  Sleep deprivation generally hurts thinking performance, but these effects are much stronger in younger adults and small or even non-existent in older adults.  (This may explain why I can play the sax in a blues band until 2am on Sunday nights and still function at work the next day.)
Of course, part of the difficulty with studying sleep in older adults is that older adults generally need less sleep than younger adults, and the older adults who get the most sleep tend to be those who are sick and whose bodies are fighting off illness.
These results do suggest, though, that the amount of sleep that older adults are getting at that phase of their lives is not a cause of cognitive decline.
A particularly interesting result is that the quality of sleep in middle age influences cognitive health in old age.  The longitudinal studies are particularly helpful for this work.  When adults in their 40s and 50s get regular sleep and allow themselves to get the roughly 8 hours of sleep they need, they show fewer signs of cognitive problems like senile dementia when they are older.  Indeed, one of the studies in this sample measured sleep quality of adults in their 40s and followed up with them 28 years later.
Putting all of this together, then, it seems that sleep is most important for current cognitive performance in younger people, and that sleep plays less of a role in thinking as we age.  Sleep in middle-aged adults is still important, though, because good sleep habits in middle-age are associated with better mental health in old-age.