Prefrontal Glx reduction during the sleep transition is associated with sleep onset latency

Understanding the neural mechanisms that facilitate the transition from wakefulness to sleep is a fundamental goal in neuroscience, yet the neurochemical dynamics underlying human sleep onset remain poorly understood. Here, we investigated whether changes in inhibitory γ-aminobutyric acid (GABA) and excitatory glutamate plus glutamine (Glx) concentrations in the medial prefrontal cortex (mPFC) are associated with sleep initiation. We leveraged the first-night effect, a transient sleep disruption induced by a novel environment, to manipulate sleep quality within participants. Twenty healthy young adults (12 females) completed two afternoon nap sessions involving simultaneous polysomnography and magnetic resonance spectroscopy of the mPFC, with sleep onset latency (SOL) serving as the primary index of sleep quality. SOL was significantly longer in the first session than in the second, confirming the first-night effect. Glx concentrations decreased during the transition from wakefulness to sleep in the second session, whereas this reduction was absent during disturbed sleep in the first session. Across sessions, greater reductions in Glx were associated with shorter SOL, and mediation analysis indicated that Glx dynamics statistically accounted for session-dependent differences in SOL. In contrast, GABA concentrations did not significantly change from wakefulness to non-rapid eye movement sleep in either session and were not associated with SOL. These findings suggest that downregulation of Glx in the mPFC is closely associated with the facilitation of human sleep onset and that a failure to reduce excitatory neurometabolite levels, rather than altered inhibition, may contribute to acute sleep-onset disturbances. Significance statement The sleep transition is a fundamental neurobiological process, yet the neurometabolic dynamics underlying this process remain elusive. By combining polysomnography with functional magnetic resonance spectroscopy while utilizing the first-night effect to manipulate sleep latency, we demonstrate that successful sleep initiation is closely associated with the downregulation of excitatory Glx, rather than changes in inhibitory GABA, in the medial prefrontal cortical region. Our results suggest that the reduction of excitatory neurometabolites, rather than the recruitment of GABAergic inhibition, is closely linked to a smooth transition into sleep. These findings offer a new neurometabolic perspective for understanding sleep disturbances and may help identify potential therapeutic targets for insomnia.