Longitudinal Sleep Spindle Maturation Trajectories and Their Association With Internalizing Problems in Adulthood.

Extensive cortical refinement from childhood to adulthood coincides with heightened vulnerability to internalizing problems, such as anxiety and depression. Thalamocortical sleep spindles may provide neurophysiological indices of such vulnerability. In this longitudinal study (2009-2023), we examined developmental trajectories of slow (11-13 Hz) and fast (13-15 Hz) spindle density across childhood (≈9.5 years), adolescence (≈16 years) and young adulthood (≈22.5 years), and tested whether individual differences in these trajectories were associated with internalizing problems during adulthood. At each wave, participants completed two nights of ambulatory polysomnography (N = 28; 19 female). Internalizing problems were assessed using five standardized self-report and external-report questionnaires. Slow spindle density declined between childhood and adulthood, with a posterior-to-anterior pattern characterized by earlier centroparietal and later frontal reductions. Fast spindle density globally increased from childhood to adolescence and then stabilized. Associations with internalizing problems were specific to slow spindle development: more internalizing problems were linked to a steeper decline in widespread slow spindle density from childhood to adolescence. Because questionnaire measures showed no evidence of developmental change in our sample, this association reflects interindividual differences rather than symptom emergence. These findings indicate that a steeper decline in slow spindle density from childhood to adolescence is associated with more internalizing problems, underscoring the importance of a developmental perspective when linking sleep spindles to emotional functioning.