This is most common in birds of prey. Researchers have found that the number of peeps increases with the number of predators around. There is also a decrease in the number of peeps when there are other birds nearby. Even fish show sleep-like activity. Even though many fish do not have eyelids, they do have a transparent mucus membrane which pulls over the eye.
All fish show sleep-like activity at some point during the day or night.
They will have a period of rest in which they relax their fins lightly on a rock or around a piece of seaweed. They tend to be less reactive to stimuli at these times as well. Some fish are even capable of secreting a mucus membrane in which they wrap themselves completely so they look as though they are in plastic. They stay in what appears to be a complete comatose state and some scuba divers even manage to pick them up without disturbing them. We can tell by our own observation that most, if not all species sleep.