Global and regional morphometric similarity in insomnia with objective short sleep duration.

Insomnia disorder is a heterogeneous psychiatric condition characterized by differences in psychological traits and neurobiological mechanisms, necessitating precise phenotyping for targeted interventions. This clinical control study, part of a two-year multicenter research project, examined differences in sleep parameters, psychological characteristics, and morphometric similarity (MS) patterns between insomnia phenotypes classified by objective total sleep time (oTST). The study enrolled 997 adult patients with insomnia disorder, of whom 270 underwent MRI scanning. Participants were categorized into insomnia with objective normal sleep duration (INSD) and insomnia with objective short sleep duration (ISSD) based on oTST measured using a wearable forehead sleep recorder. Primary outcomes included sleep parameters (e.g., wake after sleep onset and rapid eye movement percentage), psychological characteristics (e.g., rumination), and MS patterns assessed through MS mapping. Results showed that the ISSD phenotype showed shorter wake after sleep onset, lower eye movement sleep (REM) percentage, and higher non-REM stage 2 percentage, whereas the INSD phenotype exhibited greater sleep perception bias and more fragmented sleep. Additionally, ISSD showed higher global MS and distinct regional MS patterns, including lower MS in the right middle temporal gyrus and higher MS in the right postcentral gyrus. It also exhibited decoupling with the visual network and de-differentiation with the ventral attention and default mode networks. These findings reveal distinct neurobiological mechanisms underlying insomnia phenotypes and highlight the need for phenotype-based interventions.