['The present study examined the effects of indoor exposure to natural bright light on afternoon sleepiness.', 'Participants took part in 3 experimental conditions: (1) a natural bright light condition in which they carried out performance and arousal tests sitting near a window (3260.0 +/- 1812.43 lux) from 12:40 PM to 1:10 PM, (2) a nap condition in which they were provided a nap opportunity for 20 minutes from 12:45 PM, and (3) a control condition in which they performed the tests in less than 100 lux surroundings from 12:40 PM to 1:10 PM. Before and after each treatment, the same series of tests were administered.', 'A temperature- and light-controlled sleep laboratory.', 'Sixteen healthy female paid volunteers aged 33 to 43 (38.1 +/- 2.68) years.', 'Indoor natural bright light and a short nap.', 'Arousal levels were measured by the Psychomotor Vigilance Task, Alpha Attenuation Test, Karolinska Drowsiness Test, and Karolinska Sleepiness Scale. The tests were repeated every 30 minutes from 11:00 AM to 4:10 PM. Ambient light intensity was maintained at less than 100 lux, except during natural bright light exposure. Short-term exposure to natural bright light significantly improved afternoon arousal levels, as measured by the Karolinska Drowsiness Test and Alpha Attenuation Test, the effects of which continued for at least 60 minutes (1:10-2:10 PM). However, no significant differences were observed between conditions for Psychomotor Vigilance Test performance.'] Does indoor exposure to natural bright light prevent afternoon sleepiness?