1. 
   Zeitgeber
   Zeitgeber: An environmental agent or event that provides the cue for setting or resetting a biological clock. To be synchronized with our environment, we need the input of Zeitgebers. The most important Zeitgeber in nature is light. Social factors, chemical factors and activity can also serve as Zei...
   Found op http://www.medterms.com/script/main/art.html?articlekey=11607
   
   
2. 
   zeitgeber
   (from the article `nervous system, human`) ...deviates slightly from the Earth`s 24-hour cycle; a bird`s endogenous cycle is 23 hours, and the human cycle is 25 hours. In both cases the cycle ...
   Found op http://www.britannica.com/eb/a-z/z/5
   
   
3. 
   Zeitgeber
   Zeitgeber (from German for "time giver," or "synchronizer") is any exogenous (external) cue that synchronizes an organism`s endogenous time-keeping system (internal clock) to the earth`s 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light. Non-photic zeitgebers ...
   Found op http://en.wikipedia.org/wiki/Zeitgeber
   
   

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