Morning-type individuals (M-types) have earlier sleep schedules than do evening types (E-types) and therefore differ in their exposure to the external light-dark cycle. M-types and E-types usually differ in their endogenous circadian phase as well, but whether this is the cause or the consequence of the difference in light exposure remains controversial. In this study, ambulatory monitoring was used to measure 24-h light exposure in M-type and E-type subjects for 7 consecutive days. The circadian phase of each subject was then estimated in the laboratory using the dim-light melatonin onset in saliva (DLMO) and the core body temperature minimum (Tmin). On average, M-types had earlier sleep schedules and earlier circadian phases than E-types. They also showed more minutes of daily bright light exposure (> 1000 lux) than E-types. As expected, the 24-h patterns of light exposure analyzed in relation to clock time indicated that M-types were exposed to more light in the morning than E-types and that the reverse was true in the late evening. However, there was no significant difference when the light profiles were analyzed in relation to circadian phase, suggesting that, on average, the circadian pacemaker of both M-types and E-types was similarly entrained to the light-dark cycle they usually experience. Some M-types and E-types had different sleep schedules but similar circadian phases. These subjects also had identical light profiles in relation to their circadian phase. By contrast, M-types and E-types with very early or very late circadian phases showed large differences in their profiles of light exposure in relation to their circadian phase. This observation suggests that in these individuals, early or late circadian phases are related to relatively short and long circadian periods and that a phase-delaying profile of light exposure in M-types and a phase-advancing profile in E-types are necessary to ensure a stable entrainment to the 24-h day.

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