Fig. 1

Flowchart of the analyses conducted in the present study. Stage 1, the observational study. Stage 2, the Mendelian randomization study design. There are three principal assumptions in Mendelian randomization design: (A) instrumental variables (IVs) must be strongly associated with exposure, (B) IVs are not related to confounding factors, and (C) IVs affect outcome risk only through the risk factor rather than any other ways; OSA, obstructive sleep apnea; HCs, healthy controls; T2- FLAIR, T2-fluid attenuated inversion recovery; 3D-T1WI, three-dimensional T1-weighted images; rfMRI, resting-state fMRI; DTI, diffusion tensor imaging; BMI, body mass index; IVs, instrumental variables; SNP, single-nucleotide polymorphisms; LD, linkage disequilibrium