The effects of light on alertness and performance in relation to melatonin secretion
Cognitive performance and alertness are severely compromised during the night compared to the day. This lowered alertness and performance has implications for night shift workers, employers and the general public: a higher number of accidents occur during the night shift. Countermeasures suggested for these alertness and performance decrements include the use of bright light (BL), napping and caffeine. A series of experiments was carried out to test systematically whether BL did indeed improve nocturnal alertness and performance. The first experiment established that 2hrs of bright light (broad spectrum white, 02:00-04:00h, 10,000lux) did improve subjective alertness and performance. The timing, duration and wavelength of light was then investigated. It was concluded that BL given in the middle of the night (white 00:00-01:00h, 10,000lux) was more effective than light given at 20:00h or 04:00h. Light of 2hrs and 4hrs duration were more effective than 1 hr of light centred at 03:00h (white, 10,000lux). Light of shorter wavelengths appeared to be more effective than light of longer wavelengths at improving nocturnal alertness and performance (01:00-05:00h, ~300lux). The mechanism by which bright light exerted its effect was investigated and, for certain measures (subjective fatigue, feeling awake and performance on the digit symbol substitution task), the improvement seen with bright light treatment was abolished with concomitant melatonin administration (2mg, 00:15h) suggesting that for these measures bright light may exert its effect through melatonin suppression. The effects of bright light (white, 03:00-03:10h, 10,000lux) and/or caffeine (300mg) on sleep inertia after a 1hr nocturnal nap were also investigated and it was concluded that both BL and caffeine can decrease the sleep inertia effect. The results from the work contained in this thesis suggest that BL may be a useful countermeasure to lowered alertness and performance during the night.