2) They can watch the animal and notice whether its eyes are open or closed, whether it is active or lying quietly, and whether it responds to light or sound. These factors are important clues, but they often are not enough. Horses and cows, for example, rarely close their eyes whereas fish and snakes cannot close them. Yet this does not necessarily mean that they do not sleep. Have you ever seen a cat dozing with one eye partly open? Even humans have occasionally been observed sleeping with one eye or both eyes partially open. Animals don't necessarily lie down to sleep either Elephant for example often sleep standing up with their tusks resting in the fork of a tree. Finally, while sleeping animals often seem unaware of changes in the sound and light and other stimuli around them, that does not really prove they are sleeping either.
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