title: The effect of low light intensity on the maintenance of circadian synchrony in human subjects.
abstract: The light-intensity threshold for humans is not known. In past space flights owing to power restrictions, light intensities have been minimal and reported to be as low as 15 ft. c. This study was conducted to determine whether the light (L)/dark (D) environment of 16L : 8D at the relatively low light intensity of 15 ft. c. was adequate for the maintenance of circadian synchrony in human subjects. Six healthy male subjects aged 20-23 years were exposed for 21 days to a 16L : 8D photoperiod. During the first 7 days the light intensity was 100 ft. c.; it was reduced to 15 ft. c. during the next 7 days and increased again to 100 ft. c. during the last 7 days of the study. Rectal temperature (RT) and heart rate (HR) were recorded continuously throughout the 21 days of the study. In the 100 ft. c. 16L : 8D the RT and HR rhythms remained stable and circadian throughout. When the light intensity was decreased to 15 ft. c. the periodicity of the HR rhythm was significantly decreased and this rhythm showed marked instability. In contrast the period of the RT rhythm did not change but a consistent phase delay occurred due to a delay in the lights-on associated rise in RT. These divergent effects on these two rhythms in internal desynchronization and performance decrement during the 15 ft. c. exposure. The data emphasize the need for establishing accurately the minimal lighting requirements for the maintenance of circadian rhythms of humans in confined environments.
authors.affiliation: Biomedical Research Division, NASA, Ames Research Center, Moffett Field, Calif, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Winget
authors.forename: J
authors.initials: J
authors.lastname: Lyman
authors.forename: J R
authors.initials: JR
authors.lastname: Beljan
mesh_terms: D000328:Adult; D001831:Body Temperature; D002940:Circadian Rhythm / Q000502:physiology / Q000528:radiation effects*; D006339:Heart Rate; D006801:Humans; D008027:Light; D008029:Lighting; D008297:Male; D017440:Photoperiod