Influence of two different light intensities during daytime on endurance performance of handgrip exercise.

The effect of two different light intensities (dim, 50 lux and bright, 5,000 lux) on handgrip exercise in a climatic chamber (26 degrees C, 60% relative humidity) were studied in eight female subjects, aged 20-24 years. The subjects were in either the dim or bright light from 1000 hours to 1800 hours. They were then in 50 lux from 1800 hours to 2200 hours in complete darkness from 2200 hours to 0600 hours (sleep), and again in 50 lux from 0600 hours to 0800 hours. They were instructed to perform handgrip exercise with a hand ergometer until the occurrence of exhaustion from 0600 hours. The influence of dim and bright intensities during the previous daytime on the number of contractions was compared. The main findings were firstly that the mean number of contractions was 766.63 SEM 43.28 in dim and 864.5 SEM 54.76 in bright intensities (P < 0.01), and secondly that rectal temperatures were slightly but significantly lower in the bright than in the dim intensities. The reason for the reduced number of contractions in dim was discussed in terms of the establishment of a higher set-point in the core temperature in dim.