When it comes to staying mentally sharp, “sleep on it” is more than a throw-away idiom. Quite the opposite; it’s research-backed advice. Plenty of studies have shown that we absorb facts and pick up new skills more easily when we nap (or go to bed) afterwards. The cognitive benefits of studying-and-snoozing vary based on the person and cognitive challenge in question, and it's not clear why. But, it's possible that sex hormones are part of the equation. According to a not-yet-published study, napping appears to enhance a cognitive process called working memory in women, but not men.
Working memory is the process of keeping an idea or fact on deck in your-short term memory while simultaneously processing other information. We rely on this mental juggling act all the time, even when we’re waiting to make a point during a conversation. In the current study, conducted by researchers from Brock University in Canada and presented earlier this month at the American Academy of Sleep Medicine annual SLEEP conference, researchers assessed the impact of a 90-minute nap on working memory in 27 college-aged participants. And, using EEGs, they looked at sleep brain waves during the naps.
Researchers expected that working memory would improve following a nap, and that differences in performance would be explained by differences in sleep-spindle density. Sleep spindles are bursts of brain activity that precede muscle twitching. Sometimes called “sigma waves,” they’re one (of two) defining characteristics of stage-two deep sleep. Previous research has established a relationship between working memory and intelligence, as well as between IQ and spindle density. So, the Brock University researchers cut out the middleman and assumed an association between working memory and spindle density. Namely that people whose working memory improved after napping would exhibit denser spindles during stage-two sleep.
First, participants performed a task to test working memory: They viewed an image of four colored squares arranged in diamond, followed by a short delay and a second image of squares. The images were either identical or differed in terms of the squares' colors, and participants had to say which image-pairs were different. (Think: Photo Hunt)
Then, participants took 90-minute naps followed by a second working-memory test. Like before, they saw two images separated by a delay. This time, however, they had to determine the color of squares in the first image after seeing the second. Researchers repeated the experiment a week later, but eliminated the between-task nap.
Researchers’ prediction didn’t entirely bear out, as napping didn't correspond to improved task performance across all participants. But their findings revealed an unexpected trend: Napping only mattered for female participants. Women did significantly better on the second task, but only when they’d napped. Men’s scores, on the other hand, declined slightly after dozing.
Sex-based differences weren't a focal point of the study, until researchers saw the marked differences in performance and analyzed participants’ EEG tests, according to study co-author Alex Storace. “We wanted to look at their sleep architecture and say, “okay why is this happening is there something different in their sleep that’s making males perform worse and females perform better,” and so what we found was mainly regarding spindle density. And women had higher spindle density [during stage 2 of Non-REM sleep], compared to men.”
Why might women have denser sleep spindles than men? They don’t know, but suspect hormones. “There is some research suggesting that sleep architecture is different for women based on the time of the month," said Storace. "So we’re thinking that maybe that’s what explains the relationship here. We controlled for anxiety. We controlled for positive and negative [mood], but there was nothing there. It might be driven by the spindle relationship.”
To further understand the sex-differences, Storace said, they could re-do the study with a specific focus on female hormone levels.