Naturalistic studies of human light exposure

For 24 h, 10 volunteers wore an apparatus that recorded their exposure to light at eye level and at the wrist. Activity was also recorded from the head, both wrists, and an ankle. Most of the subjects were exposed to daylight illumination intensities for only brief and scattered episodes during the 24 h. Some experienced the brightest illumination in the morning and others in the evening. Illuminations at eye level and at the wrist were correlated 0.76, while eye-level illumination was correlated 0.25, 0.44, 0.39, and 0.44 with head, wrists, and ankle activity, respectively. Because human biologic rhythms are probably well synchronized only by illumination approaching daylight intensities, inadequate illumination could be a source of sleep disturbance, chronobiologic disorders, or depression.