Bedroom nocturnal Ta and humidity were significantly higher in summer and daytime illumination was significantly higher in spring than in the other seasons. Sleep efficiency was worst in summer due to the increased number and duration of nocturnal awakenings. However, a significant difference was not found in the subjective evaluation of sleep among the 4 seasons. The subjects felt warmer and reported more subjective wakefulness in spring than in the other seasons; however, their sleep quality was not diminished. The correlations between the sleep parameters and environmental factors such as temperature, humidity, and illuminance levels measured at the same time showed that increased lighting level before the sleep prolongs the bedtime and wake time after sleep onset, and became earlier wake-up time. Increased ta, humidity, and lighting level during the sleep period increased wake time after sleep onset and impaired the sleep quality.