Bright light has a role in natural coordination of mammalian circadian and seasonal rhythms. In humans, the light intensity must probably exceed 2000 lux to be optimal. Natural light exposures of 10 healthy adults were measured over a 24-hour period, using forehead illumination transducers connected to a portable computer. The subjects varied markedly in duration and timing of exposures to light greater than 2000 lux. On average, the subjects experienced bright light for only 90 minutes per day, less than the 3-8 hours of bright light necessary to maximally synchronize human circadian rhythms. These results suggest that natural and artificial light exposure for many Americans may be suboptimal for circadian and seasonal synchronization.