Single-dose bright light and/or caffeine effect on nocturnal performance.

BACKGROUND

The impact of the separate and combined effects of a 1-h exposure to bright light (approximately 3000 lx) and a 200-mg dose of caffeine on nocturnal performance was studied during a simulated shift-work schedule beginning 1730 in the evening and ending 1000 the next morning.

HYPOTHESIS

Light and caffeine exposure were expected to improve nocturnal fatigue degradation.

METHODS

There were 11 subjects tested under 4 treatment conditions: 1) 1 h Dim Light-Placebo; 2) 1 h Bright Light-Placebo; 3) 1 h Dim Light-Caffeine; 4) 1 h Bright Light-Caffeine. Exposure to the light occurred between 0130 and 0230 hours. Caffeine or placebo was administered at 0140 hours.

RESULTS

Choice Reaction Time (RT) recorded during the four post-treatment sessions were shorter for the Bright Light-Caffeine, Bright Light-Placebo, and Dim Light-Caffeine conditions than for the Dim Light-Placebo condition. During the sessions beginning 0430 and 0830 hours, the shortest RT was recorded for the Bright Light-Caffeine treatment. The largest number of trials without false alarms per session for the working memory task (letter cancellation) was found for the Bright Light-Caffeine condition. Exposure for 1 h to 3000 lx reduced melatonin concentration between 42-47% from 0230 to 0410 hours. A 200-mg dose of caffeine also reduced melatonin levels, although to a lesser degree than 1 h exposure to 3000 lx.

CONCLUSION

Although 1 h exposure to bright light at 0130 hours combined with a 200-mg dose of caffeine maintains performance throughout the remainder of the night/early morning, a 1-h exposure to bright light without the caffeine may actually degrade performance.