biological clock

biological clock
    n 1: an innate mechanism in living organisms that controls the
         periodicity of many physiological functions
    

45 Moby Thesaurus words for "biological clock":
      anima, animating force, atman, bathmism, beating heart, biorhythm,
      blood, breath, breath of life, divine breath, divine spark,
      elan vital, essence of life, force of life, growth force, heart,
      heartbeat, heartblood, impulse of life, inspiriting force, jiva,
      jivatma, life breath, life cycle, life essence, life force,
      life principle, life process, lifeblood, living force, pneuma,
      prana, seat of life, soul, spark of life, spirit, vis vitae,
      vis vitalis, vital energy, vital flame, vital fluid, vital force,
      vital principle, vital spark, vital spirit