Are Age‐Related Differences in Sleep Associated With the Different Trajectories of Fluid and Crystallized Intelligence Across the Lifespan?

A common misconception is that, with age, cognition deteriorates across all domains. Instead, certain aspects are negatively impacted, while others are relatively spared, or, continue to improve. Specifically, fluid intelligence (i.e., problem-solving skills) peaks in early adulthood and declines, whereas crystallized intelligence (i.e., ability to use and remember facts) increases into older age. Sleep supports cognition, but, sleep quality/quantity are reduced with age. It is unknown how age-related differences in sleep contribute to the preservation or decline of different domains of intelligence. Here, we investigated if sleep quantity/quality were associated with the trajectories of fluid and crystallized intelligence with age. In Study 1, N = 12,182 participants completed a sleep questionnaire and the Cambridge Brain Sciences (CBS) cognitive test battery. In Study 2, 60 healthy adults underwent polysomnographic recording, 10 days of actigraphy and the CBS tests. In Study 1, subjective sleep assessments from the general population revealed that crystallized intelligence was preserved whereas fluid intelligence declined as a function of age. However, sleep did not account for these different trajectories. In Study 2, objective sleep assessments in a sample of very healthy individuals, revealed the opposite pattern; crystallized intelligence declined, whereas fluid intelligence was relatively preserved. Age-related differences in total sleep time, wake after sleep onset, the regularity and strength of the sleep-wake cycle accounted for the different trajectories of crystallized and fluid intelligence. Thus, controlling for confounding variables and using objective sleep assessments shows that regular, higher-quality sleep is protective for fluid intelligence with increasing age.