PIP: This article explores the influences of light on human health and suggests that exposure to artificial light may have harmful effects. The effects of ligght on mammalian tissue are either direct or indirect, depending on whether the immediate cause is a photochemical reaction within the tissue or a neural or neuroendocrine signal generated by a photoreceptor cell. Light exerts an indirect effect on the ovaries of rats and this effect is mediated by photoceptive cells in the retina. The light cycles involved in night and day and changing day lenght appear to be associated with rhythmic changes in mammalian biological functions such as body temperature. Light levels and rhythms also influence the maturation and subsequent cyclic activity in the gonads of mammals, with the particular response seemingly dependent on whether the species ovulated once a year or at regular intervals. Ovulation can be accelerated in diurnally active, monestrous animals by exposing them to artificially long days. Pineal activity in rats can be suppressed by exposing the animals continuously to light. Such findings on the multiple and disparate effects of light suggest the view that health considerations should be incorporated into the design of light environments. The illumination provided by artificial indoor lighting is often less than 10% of the light normally available outdoors. It is urged that decisions on lighting be based on knowledge of man's biological needs as well as economic and technoloical considerations.