This　field　study　aimed　to　investigate　naturally　ventilated　bedroom　environment　and　its　effects　on　subjective　perception　and　sleep　quality.　Totally,　104　healthy　subjects　living　in　urban　areas　of　Beijing　participated　in　the　study　for　one　night　during　transition　seasons.　Bedroom　environment　parameters,　including　temperature,　relative　humidity,　and　CO2　concentration,　were　recorded　before　and　during　sleep.　Objective　sleep　quality　was　measured　by　Fitbit　Alta　2,　a　wrist-type　actigraphy　sensor.　Subjective　assessments　were　collected　by　paper-based　questionnaires　on　sleep　quality　and　environmental　perceptions.　The　results　showed　that　neutral　temperature　for　waking　state　(before　sleep)　was　estimated　to　be　23.8°C　while　for　sleep　state　it　was　26.5°C.　Furthermore,　pre-sleep　thermal　sensation　vote　was　found　to　be　positively　correlated　with　deep　sleep　percentage.　Indoor　air　quality　was　correlated　with　sleep　quality　as　indicated　by　statistically　significant　correlations　between　odor　intensity　assessment,　air　quality　acceptability,　average　nightly　CO2　concentration,　and　measures　of　sleep　quality.　For　naturally　ventilated　bedrooms　during　transition　seasons　with　a　mild　outdoor　climate,　present　findings　suggest　that　a　bedroom　with　slightly　warm　pre-sleep　environment　than　neutral,　and　with　high　ventilation　as　indicated　by　low　indoor　CO2　concentration,　could　be　beneficial　for　sleep　quality　of　residents.