Closed-Loop Spindle-Targeted Stimulation Enhances Consolidation of Probabilistic Sequence Learning in Balanced Bilinguals

Sleep spindles have been implicated in consolidating motor and cognitive skills, yet causal evidence for their role in probabilistic sequence learning among bilingual adults is limited. We combined real-time spindle detection with phase-locked auditory stimulation in a double-blind, sham-controlled, within-subject crossover study (N=36) of balanced Dutch–English bilinguals trained on a probabilistic serial reaction time task. During N2 sleep, brief 50 ms pink-noise bursts were delivered at the up-state of detected central spindles (Cz) in the Stimulation night and withheld in the Sham night. Relative to Sham, Stimulation increased fast spindle density (13–16 Hz) at central sensors by 18% and enhanced overnight improvement in sequence-specific performance by 15.6 ms (SE 4.7 ms, p=.002, d=0.64) without affecting accuracy or macro-architecture. Effects were strongest in participants below the median baseline and generalized across minimal language-context cues (L1↔L2). Mediation analyses indicated that the spindle-density change partially accounted for behavioral gains (β=0.27, p=.01). Findings provide causal support that augmenting spindles during N2 selectively strengthens probabilistic sequence representations in bilingual adults, and they motivate translational protocols for skill consolidation in multilingual classrooms and rehabilitation.