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.