CIRCADIAN rhythms have a period of about 24 hours, corresponding roughly to the astronomical day length which depends on the rotation of the Earth on its axis. It is the best studied biological cycle due to the relative ease of gathering sufficient data over multiple 24-hour periods to be statistically significant. Thus, if a researcher has a one year grant, they can obtain 365 days of observations for their study. Light (Photobiology) and temperature changes have been cited as possible synchronizers or Zeitgebers of these daily cycles. Green plants use light to produce chlorophyll. The byproduct of plant respiration during the day is oxygen. At night they actually use oxygen and "exhale" carbon dioxide. Some plants open their leaves during the day and close them at night. Others bloom only at night (night-blooming Cereus). Certain animals are nocturnal, usually only coming out at night. Bats, raccoons and hedgehogs are good examples. The male nightingale sings his best in the evenings. Some human circadian rhythms are : The "normal" sleep(night) / wake (day) cycle Body temperature Pulse / heart rate Blood pressure

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