ABSTRACT
Under most circumstances, human daily rhythms, and those of the plants and animals that share the world, are synchronized, or entrained, by the daily rising and setting of the sun. Daily light provides precise time cues. A German word, Zeitgeber, which means "time-giver", denotes the environmental signals that provide time cues—lighting, temperature, food availability, sound, social factors, and so on. As a result of the daily and seasonal cycles of sunlight, there are less precise daily and seasonal temperature cycles that provide potential Zeitgebers. Social factors have also been proposed as Zeitgebers. Sometimes organisms kept in exotic short cycles displayed a 24.0-hour cycle. This phenomenon is called frequency demultiplication. In a light–dark cycle, the length of the light-time, the day-length, is the photoperiod and the length of the dark-time, the night-length, is the scotoperiod. A skeleton photoperiod is created by using a pulse of light to represent dawn and another pulse of light to represent dusk.
