I n biology, sunlight is just one distinction between night and day.
Plants churn out more chlorophyll during daytime.
Even in a lowly blue-green alga, cell division, nitrogen fixation, photosynthesis and respiration all reflect time of day or night. In fact, creatures have apparently been responding to light and darkness ever since multi-cellular organisms appeared about 700 million years ago. The idea that mammals had a built-in clock once seemed exotic enough. Now it's clear that mammals, plants, many fungi and even blue-green algae also have separate clocks in most of their cells. Likewise, research groups lead by Michael Menaker at the University of Virginia and Hajime Tei at the University of Tokyo have shown that circadian oscillations continue for a few days in slices of rat liver or lung grown in tissue culture.