- People with a genetic disposition for taking naps have a larger brain volume than others, according to new research.
- With brain volume often decreasing with age, the study shows that people with this genetic predisposition exhibit fewer years of brain aging than non-nappers.
- Whether the study reveals a causal connection between napping and brain health remains unclear, but it nonetheless offers a new exploration angle.
While some research indicates that short naps may boost cognition, a direct causal link between napping and brain health remains unidentified.
A new study from researchers at University College London in the U.K., the University of the Republic in Uruguay, and the Broad Institute in Massachusetts uses an analysis technique to tease out causal relationships.
The new study investigates a possible association between a genetic disposition toward napping and brain volume, an indicator of brain health.
The authors of the study analyzed data from 378,932 participants in the UK Biobank. Individuals were an average of 57 years old, with their ages ranging from 40 to 69.
Among these people, the researchers looked for individuals with the 92 genetic variants previously identified as being associated with habitual napping. They also assessed brain volume, hippocampal volume, reaction time, and visual memory for people with these genetic markers. The Mendelian randomization technique revealed the links the study reports.
The researchers found that people with these genetic characteristics were more likely to have a higher brain volume. A loss of brain volume is associated with neurodegeneration in the form of atrophy caused by cell death, so a robust brain volume is considered a sign of health.
The study is published in Sleep Health.
Naps and brain function
Napping is an ingrained activity in many cultures, with millions of people around the world nodding off for a short rest during the day. While some people may feel guilty about taking a snooze during the day, there is research that suggests naps have value.
A 2016 study found that there are cognitive benefits to brief naps of 30 minutes to 90 minutes. People can wake up from such time-outs more alert and sharp.
However, the same research suggests that napping for longer than that can result in cognitive problems.
The relationship between naps and long-term cognitive health, however, remains unclear.
One study found that cognitively healthy nappers are at higher risk of Alzheimer’s. Another study found the opposite.
Causal relationship or just another association?
The study’s lead author, Valentina Paz, a Ph.D. candidate and assistant professor of psychology at the University of the Republic in Uruguay, told University of College London News:
“This is the first study to attempt to untangle the causal relationship between habitual daytime napping and cognitive and structural brain outcomes. By looking at genes set at birth, Mendelian randomization avoids confounding factors occurring throughout life that may influence associations between napping and health outcomes.”
In an interview with Medical News Today, Dr. Paz said her study “specifically revealed a 15.8 cm3 [cubic centimeters] increase in total brain volume with more frequent daytime napping.”
Dr. Paz described this as “approximately equivalent to 2.6 to 6.5 years of difference in aging.”
She cautioned, “More work is needed to disentangle this association.”
Mendelian randomization is defined as a “use of genetic variation to address causal questions about how modifiable exposures influence different outcomes.”
But does Mendelian randomization prove causality or present a different way of identifying associations?
As Dr. Penelope Lewis, Ph.D., a professor at the School of Psychology at the University of Cardiff, noted to MNT:
“I do wonder if it is correct to claim that they show a causal relationship between napping and brain size — surely it is just a correlation?”
The study did not examine direct cognitive benefits of naps, and in fact, did not assess nappers directly at all — it was concerned only with people with a genetic predisposition to napping.
As such, Dr. Rebecca Spencer, Ph.D., a professor of neuroscience at the University of Massachusetts Amherst, said she felt the study did not capture many critical details.
Dr. Spencer said she took issue with the lack of data regarding participants’ napping habits. The subjects had been asked in the UK Biobank survey if they napped during the day, with three possible responses: “never/rarely,” “sometimes,” or “usually.”
“This is flawed, Dr. Spencer said. “It is subjective.” She pointed out the study does not specify a range of times for which nap habits are reported. “Past week? Past month? Past year? Past decade? [Their] lifetime?” she questioned.
Dr. Spencer also noted the lack of a standardized definition of a nap — “eyes close on the train ride home from work versus curl up in bed, versus unintentionally fall asleep at the dinner table.”
There is also no standard in the study for the length of a nap.
How naps might help brain health
“Sleep has been shown to be important for flushing toxins out of the brain, and thus maintaining health,” said Dr. Lewis. “Loss of slow-wave sleep across age has been shown to predict cortical atrophy.”
“I would speculate that if the naps contain slow-wave sleep, this could be helping to preserve against cognitive aging and atrophy,” Dr. Lewis said.
While dreams are typically associated with REM sleep and not deep, slow-wave sleep, recent research finds that one may experience dreaming even during slow-wave sleep.
Time for a nap
Dr. Paz said, “I believe that our findings further our understanding of the relationship between daytime napping frequency and brain health, even though more work is needed on this topic.”
While Dr. Lewis credited the study with providing new information relating to what must be a continuing investigation of the effects of napping and sleep in general.
Still, she noted, “I would be cautious about saying that napping is good for brain health, as napping can lead to poor overnight sleep.
“I would say that getting a good amount of sleep every 24 hours is clearly important for health in general,” Dr. Lewis concluded.
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