ObjectiveTo investigate the characteristics of polysomnography （PSG） in depression patients complicating moderate-to-severe obstructive sleep apnea hypopnea syndrome （OSAHS）. MethodsA retrospective analysis was conducted on the outpatients， inpatients and physical examination population who completed overnight PSG monitoring in the sleep medicine center of Suzhou Guangji Hospital from December 2017 to October 2019. Four groups of subjects were finally enrolled， including depression patients with moderate-to-severe OSAHS （n=31）， depression patients without OSAHS （n=79）， moderate-to-severe OSAHS patients （n=96） and normal control group （n=32）. The sleep process related indicators （total sleep time， sleep latency， number of awakenings）， sleep structure related indicators （N1， N2， N3， percentage of REM sleep， REM latency， REM sleep duration）， sleep-related respiratory variables （oxygen reduction index） and other polysomnographic parameters of the four groups were compared. ResultsIn terms of sleep process， the total sleep time， sleep latency and number of awakenings yielded significant differences among the four groups （F=2.874， 3.959， 12.291， P<0.05 or 0.01）. In terms of sleep structure， the percentage of total sleep time in N2 and N3 stages demonstrated significant differences among the four groups （F=13.885， 48.013， P<0.01）. The REM latency， REM sleep duration and percentage of REM sleep manifested significant differences among the four groups （F=41.492， 11.827， 10.552， P<0.01）. In terms of sleep-related respiratory variables， the oxygen reduction index exhibited significant differences among the four groups （F=170.585， P<0.05）. ConclusionDepression patients complicating moderate-to-severe OSAHS suffer from severe sleep process and structural disturbances， accompanied by quite frequent and severe sleep-related respiratory events.