Sunlight is the origin of nearly all the metabolic energy that drives life processes, and plants and algae are by and large responsible for the conversion of solar energy into chemical forms that are used by all organisms. These simple truths are perhaps obvious to plant biologists, but I think they merit enunciation every once in a while, if only to remind zoologists that, ultimately, plants rule. Even the fossil fuels that support much of modern human culture are owed to the photosynthetic activity of plants. Of course, plants have not evolved to photosynthesize on behalf of the less self-reliant inhabitants of the planet, but rather to provide for their own nourishment, growth, and development. Thus, virtually every aspect of plant life, from germination through reproduction and senescence, is affected by light. The biology of many nonphotosynthetic organisms is also linked to sunlight. The reflective and refractive properties of light are visually perceived by many animals so that they may gauge within their varied and changeable environments the availability of food, water, shelter, and even reproductive partners, as well as evade the threat of predators and other dangers. Vertebrate sleep cycles, and sleep-like states even in invertebrates, are also linked to the sun’s daily periodicity. Circadian rhythms, in fact, appear to be basic to organisms as diverse as fungi, plants, and mammals.
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Harry B. Smith (2000) studied this question.