Abstract:Living in isolation from time cues under relatively high and low light intensities for a total (on average) of 24 days, 18 subjects estimated the passage of time by “producing” short (10 to 120 seconds) and long (1h) intervals throughout the experiments. The 1h productions were independent of light intensity and highly positively correlated with the duration of wake times. The short-interval productions were markedly increased under high light intensity. In a subsample of 6 subjects, the interaction between effects of body temperature and light condition on 10-second production was analyzed. Productions were negatively correlated with body temperature. In both dim and bright light, productions decreased by a factor of 0.7 per °C. In bright light, production was increased by a factor of 1.2 relative to dim light. This effect was not mediated by body temperature, which itself was on average slightly increased in bright light. Since subjective time is slowed by bright light, objective time seems to pass faster in bright light.