Nearly every physiological process has a circadian rhythm lasting about 24 hours. In the elderly and particularly in those with dementia these rhythms become unstable and have a reduced amplitude. This review article describes the fundamental changes that occur in the suprachiasmatic nucleus, which is the centre of the circadian system, and explains how these changes interfere with the health, sleep, mood and cognitive performance of the elderly and the elderly with dementia and affect their well-being. Apparently, the circadian system of the elderly receives fewer stimuli since the elderly are less frequently exposed to bright light, produce less melatonin, engage in less physical activity and are less affected by somatosensory and thermoreceptive stimuli. The review article focuses on the problems arising from reduced exposure to bright light. That seems to gradually inactivate the scn neurons. It has been shown that exposure of aged animals to extra light reactivates their neurons and revitalises their poorly functioning circadian systems. In the case of healthy old people and old people with dementia extra light appears to relieve the above mentioned negative effects of their malfunctioning circadian systems. These findings indicate the preservation of the plasticity of the circadian rhythm in the elderly. They also emphasise the need for longitudinal placebocontrolled randomised investigations into the benefits that daily supplements of bright light can bestow in the elderly in terms of improved health and well-being.