title: Effects of day-time exposure to different light intensities on light-induced melatonin suppression at night.
abstract: BACKGROUND
  Bright nocturnal light has been known to suppress melatonin secretion. However, bright light exposure during the day-time might reduce light-induced melatonin suppression (LIMS) at night. The effective proportion of day-time light to night-time light is unclear; however, only a few studies on accurately controlling both day- and night-time conditions have been conducted. This study aims to evaluate the effect of different day-time light intensities on LIMS.
  
  
  METHODS
  Twelve male subjects between the ages of 19 and 23 years (mean ± S.D., 20.8 ± 1.1) gave informed consent to participate in this study. They were exposed to various light conditions (<10, 100, 300, 900 and 2700 lx) between the hours of 09:00 and 12:00 (day-time light conditions). They were then exposed to bright light (300 lx) again between 01:00 and 02:30 (night-time light exposure). They provided saliva samples before (00:55) and after night-time light exposure (02:30).
  
  
  RESULTS
  A one-tailed paired t test yielded significant decrements of melatonin concentration after night-time light exposure under day-time dim, 100- and 300-lx light conditions. No significant differences exist in melatonin concentration between pre- and post-night-time light exposure under day-time 900- and 2700-lx light conditions.
  
  
  CONCLUSIONS
  Present findings suggest the amount of light exposure needed to prevent LIMS caused by ordinary nocturnal light in individuals who have a general life rhythm (sleep/wake schedule). These findings may be useful in implementing artificial light environments for humans in, for example, hospitals and underground shopping malls.
authors.affiliation: Faulty of Design, Kyushu University, 4-9-1 Shiobaru, Fukuoka city, Minami-ku, Japan. kozaki@design.kyushu-u.ac.jp.
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Kozaki
authors.affiliation: Graduate School of Design, Kyushu University, 4-9-1 Shiobaru, Fukuoka city, Minami-ku, Japan.
authors.forename: Ayaka
authors.initials: A
authors.lastname: Kubokawa
authors.affiliation: Graduate School of Design, Kyushu University, 4-9-1 Shiobaru, Fukuoka city, Minami-ku, Japan. r.taketomi.339@s.kyushu-u.ac.jp.
authors.forename: Ryunosuke
authors.initials: R
authors.lastname: Taketomi
authors.affiliation: Graduate School of Design, Kyushu University, 4-9-1 Shiobaru, Fukuoka city, Minami-ku, Japan. hatae.keisuke.725@s.kyushu-u.ac.jp.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Hatae
mesh_terms: D000328:Adult; D002940:Circadian Rhythm / Q000528:radiation effects*; D006801:Humans; D008027:Light; D008029:Lighting / Q000379:methods*; D008297:Male; D008550:Melatonin / Q000032:analysis / Q000378:metabolism*; D012463:Saliva / Q000737:chemistry; D055815:Young Adult
chemical_list: D008550:Melatonin