DESCRIPTION (adapted from investigator's abstract): The investigator's long-term goal is to understand the basis of and develop effective therapies for chronic sleep disturbances in older adults. One common sleep disorder in older adults is advanced sleep phase, accompanied by sleep maintenance insomnia and early morning awakenings. This can shorten the total sleep time and lead to daytime fatigue and impaired performance. The advance in sleep is associated with an advances in the timing of the circadian core body temperature rhythm which suggests an advance in the timing of the circadian clock. The cause of this advance is unknown. Preliminary date from the investigators laboratory suggests that elderly subjects do not phase delay following exposure to 4000 lux for 3 hours before the temperature minimum, a time that usually does delay the rhythm in younger adults. The first goal of this application is to understand the mechanism underlying the age-related change in responsiveness of the clock to light. The second goal is to assess whether it is possible to compensate for age-related change in the responsiveness of the aging circadian clock to light by either increasing the intensity of the light exposure or by pharmacological treatment with the calcium channel antagonist nimodipine. The proposed experiments will provide a vast amount of data in which to better understand the effect of age on circadian rhythms and sleep and lead to improved treatments for circadian rhythm and sleep disorders in older adults.