Brain areas involved in sleep and biological rhythm regulation are sensitive to light and temperature, which in an evolutionary sense are the oldest cyclically varying physical aspects of the environment. Hypofunctionality of these areas is involved in nocturnal restlessness in demented elderly – a primary risk factor for transfer to a nursing home. Brain areas involved in sleep and biological rhythm regulation are sensitive to light, which in an evolutionary sense is the oldest cyclically varying physical aspect of the environment. In an attempt to reactivate the biological clock, 189 demented elderly were treated and tracked for up to 3 ½ years in the first ever long-term multicenter randomized clinical trial on the effect of light. The originality and importance of the study caught the eye of Nature and Science already before it was completed. Published recently in one of the most important clinical journals (JAMA 2008;299:2642-55)(Riemersma-van der Lek et al., 2008) the results are striking, with great societal consequences and international appreciation (e.g. assigned as ‘recommended paper’ at the Alzheimer Research Forum). Increasing the amount of light reduced nocturnal restlessness by 9% per year; cognitive impairments by 5%, depressive symptoms by 19% and the deterioration in functional abilities (activities of daily living) by 53%. Without a doubt these effects can compete with the effects of individual symptomatic pharmacological treatments for each of these disorders, but without the side effects. Indeed, everyday health problems even reduced. The results and their publication in one of the most prominent medical scientific journals are an international breakthrough regarding the application possibilities of chronobiology and have great societal relevance. They pave the way for implementation of adequate - high intensity - environmental natural or electric light in care centers for the elderly. This is a relatively inexpensive measure that will lead to an improvement in the quality of life of many elderly people and a decrease of the burden of care for many carers. We are presently evaluating the effect of long-term skeleton photoperiod application in early stage Alzheimer’s disease.