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All animals, including single-celled organisms, alternate between waking and resting states. But this does not mean that they sleep according to the human concept of sleep. As they do not have eyelids, fish cannot close their eyes and their period of rest consists of a state of barely apparent immobility, in which they maintain balance through very slow movements of their fins. Some species rest lying on the bottom of the sea or river, others hide among dense vegetation or in holes. Without rest, the fish would die of fatigue. Even the shark, known as a tireless swimmer, has its rest periods.