Effects of seasonal illumination and thermal environments on sleep in elderly men

• Bedroom Ta and humidity was significantly highest in summer.
• Increased number and duration of awakenings reduced sleep efficiency in the summer.
• No difference was found in the subjective evaluation of sleep among the 4 seasons.
• Increased lighting level before sleep prolongs the bedtime and increased wake time.
• Higher Ta, humidity, or illuminance during sleep increased wake time for elderly.

The purpose of this study was to investigate the effect of changes in the seasonal thermal environment and illuminance on sleep in older men by using actigraphy. Eight healthy male volunteers with a mean age of 64 ± 1 years served as subjects. The measurements were obtained in 4 consecutive seasons: spring, summer, autumn, and winter. The activity level and illuminance were monitored using a wrist actigraph system with illuminance meter for 5 consecutive days. Sleep parameters were determined using an actigraph-based, sleep-wake identification algorithm. The temperature and humidity in the bedroom of the subjects' homes were measured continuously for 5 days. During the actigraphic measurement, skin temperature and the temperature and humidity of the microclimate were measured continuously for 2 nights. Bedroom nocturnal Ta and humidity was significantly higher in the summer than in the other seasons. Sleep efficiency was worst in the 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 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 wake-up time earlier. Increased ta, humidity, and lighting level during the sleep period mount up the wake time after sleep onset and impaired the sleep efficiency index.