This study will apply a comprehensive tools that integrates neuroimaging, psychological evaluation, and sleep monitoring through 18F-MPPF PET/MR, neuropsychological tests, and polysomnography (PSG) to explore the neurobiological mechanisms underlying transcranial alternating current stimulation (tACS) for depressive disorders, mainly focusing on the serotonergic system revealed by Serotonin-1A (5-HT1A) receptor.