Sleep Quality, Not Exercise, Tracks Working Memory in Young Adults, Study Finds

When it comes to keeping the mind sharp in early and middle adulthood, the advice most people hear is familiar: exercise more, sit less, sleep well. But a new study from King Saud University in Riyadh, published in the Journal of Adult Development, suggests that this trio of lifestyle factors does not act on the brain as a single, undifferentiated lever. Instead, the researchers report, different habits appear to touch different cognitive systems—and the one that mattered most for a core mental faculty, working memory, was not physical activity at all. It was the quality of a person’s sleep.

The study, conducted by Sumyah Alnajashi and Albandri Alotaibi of the Department of Psychology at King Saud University, set out to answer a question that has long frustrated cognitive scientists. Decades of research have linked lifestyle factors to cognitive functioning, yet the relative contribution of each habit to distinct cognitive constructs remains murky—particularly in non-Western populations, who are dramatically underrepresented in the literature. Most large studies of sleep, sedentary behavior, and cognition have been conducted in North America, Europe, or East Asia, leaving open the possibility that findings do not generalize to Arabic-speaking adults and other groups whose daily routines, sleep patterns, and activity profiles may differ.

To probe these relationships, the team recruited 235 Arabic-speaking adults aged 18 to 46, with a mean age of about 26.5 years; 172 of the participants were women. Each participant completed a battery of online cognitive tasks alongside self-report questionnaires covering sleep and physical activity. The task battery was deliberately chosen to tap two theoretically distinct cognitive domains. The Stroop task, in which people must name the ink color of a conflicting color word, and the Flanker task, in which participants respond to a target flanked by distracting letters, both measure the efficiency of processing and inhibitory control. The Corsi block-tapping task, which requires reproducing sequences of illuminated blocks, and a digit-updating task, which demands continuous mental manipulation of numbers, instead probe working memory—the mind’s capacity to hold and update information over seconds.

A key methodological strength of the study lies in how it quantified performance on the speeded tasks. Rather than relying on raw reaction times or accuracy alone, the researchers applied the EZ-diffusion model, a simplified version of the diffusion decision model widely used in cognitive psychology. This model decomposes task performance into distinct parameters, including the drift rate—the rate at which the brain accumulates evidence toward a decision. Drift rate is increasingly regarded as a cleaner indicator of what researchers call cognitive efficiency: how quickly and effectively the underlying decision system processes information, independent of response caution or motor speed. By deriving drift rates from the Stroop and Flanker tasks, the team built a latent factor of cognitive efficiency that was statistically separated from the working memory factor, which was indicated by Corsi span and digit-updating performance.

The structural equation model that tied these cognitive factors to lifestyle predictors and covariates—including age, sex, device type, and socioeconomic status—fit the data exceptionally well, with a nonsignificant chi-square statistic, a comparative fit index of 1.000, and near-zero error indices. Against that well-fitting backdrop, the results were strikingly asymmetric. Sleep quality significantly predicted working memory, with a standardized coefficient of 0.265, while age showed a negative association of comparable magnitude, consistent with the well-documented gradual erosion of working memory across adulthood. Sedentary behavior showed a marginal positive association with working memory that just missed conventional significance thresholds. Together, the predictors explained 21 percent of the variance in working memory—a substantial share for individual-differences research in this domain.

Cognitive efficiency told a different story. None of the lifestyle predictors significantly explained variance in this factor, which accounted for only about 5.7 percent of its variance. Sedentary behavior again showed a marginal trend, this time in the negative direction, but the overall picture was one of silence: neither sleep quality, nor sleep duration, nor physical activity reliably tracked how efficiently young and middle-aged adults accumulated evidence in speeded decision tasks. Sleep duration and physical activity were nonsignificant for both cognitive factors, a result that may raise eyebrows given the enormous literature linking exercise to brain health.

The authors argue that this dissociation is itself the headline finding. Much previous work has treated cognition as a monolith, aggregating heterogeneous tasks into a single composite score. But cognitive psychology has long distinguished between constructs such as processing efficiency, inhibitory control, and working memory capacity, and latent-variable research shows that these constructs correlate only partially. If lifestyle factors differentially touch these systems—as the present results suggest—then studies that collapse them together may dilute or obscure real effects. Sleep quality might matter for the maintenance and updating of memory representations while leaving the raw speed of evidence accumulation largely untouched, at least in healthy young adults whose sleep problems are typically mild rather than chronic and severe.

The findings also resonate with a broader shift in sleep science toward multidimensional sleep health, in which quality, continuity, and timing are treated as separable dimensions with separable consequences. Prior studies have found that subjective sleep quality predicts cognitive performance in healthy young adults even when objective sleep measures do not, and that poor sleep quality is associated with reduced visual working memory capacity specifically. The new results extend this pattern to an Arabic-speaking sample, addressing a genuine gap in the literature. They also align with evidence that subjective sleep measures can be more predictive of global cognitive function than objective ones in some contexts, hinting that how people perceive their sleep may capture something—perhaps sleep fragmentation, or the distress associated with poor rest—that actigraphy alone misses.

As for the null results on physical activity and sleep duration, the researchers are careful about interpretation. The study is cross-sectional, meaning it captures a snapshot rather than tracking change over time, and it relies on self-report measures of lifestyle, which are noisy and subject to bias. The sample, while valuable for its cultural and linguistic reach, skewed young and female, and the range of physical activity and sleep duration may have been restricted in ways that limited statistical power. It is also possible that the cognitive benefits of exercise documented in older adults—where meta-analyses consistently show protective effects against decline—simply have not yet had time to manifest in a sample whose mean age is in the mid-twenties. The marginal sedentary behavior effects, positive for working memory and negative for efficiency, echo a growing literature suggesting that sedentary time is not uniformly harmful and that the type of sedentary activity, whether cognitively engaging or passive, may matter more than total hours.

What the study offers, ultimately, is a sharpened target. If the results hold up in longitudinal and intervention designs, sleep quality stands out as a potentially modifiable factor associated with working memory in adulthood—a faculty that underpins reasoning, planning, and everyday problem-solving and that begins its slow decline earlier than many realize. In a world where sleep is routinely sacrificed to screens, work, and social demands, the message from Riyadh is a quietly provocative one: the habits that protect the mind’s capacity to juggle information may be won or lost not in the gym but in bed, and researchers who want to understand how lifestyle shapes cognition would do well to stop averaging across mental faculties that behave in fundamentally different ways.

Subject of Research: Associations of sleep quality, sedentary behavior, and physical activity with cognitive efficiency and working memory in adults

Article Title: Differential Associations of Sleep, Sedentary Behavior, and Physical Activity with Cognitive Efficiency and Working Memory in Early and Middle Adulthood

Article References: Differential Associations of Sleep, Sedentary Behavior, and Physical Activity with Cognitive Efficiency and Working Memory in Early and Middle Adulthood. (n.d.). https://doi.org/10.1007/s10804-026-09580-y

Image Credits: AI Generated

DOI: 10.1007/s10804-026-09580-y

Keywords: sleep quality, working memory, cognitive efficiency, drift rate, EZ-diffusion model, sedentary behavior, physical activity, structural equation modeling, early adulthood, middle adulthood, cognition, Journal of Adult Development