ABSTRACT
Many day-active types of animals retreat to a secure refuge at night, where they rest and sleep from dusk to dawn. Moreover, as the length of the night changes during the course of the year, they adjust the duration of their nightly period of rest and sleep so that it is longer in winter and shorter in summer (1). In a natural environment, humans may have once behaved similarly; in the modem, urban environment, however, humans use artificial light to escape the confines of the solar day and extend their waking activities into the first hours of the night. This practice delays the onset of their nightly period of rest and sleep and keeps it short and unvarying year-round.
