With other species, the evidence is less precise and dramatic. By following that line of thinking, it seems logical that the mammals that sleep the most have more developed brains and intelligence levels, but this is not demonstrated as accurately in mammals as it is in humans. However, researchers did not stop with this information.
They conducted studies and pursued modalities in research that indicated a direct correlation between the metabolic rate and sleep patterns in animals. For instance, animals that have a large body size need more non-REM sleep in order to help them conserve their energy. On the other hand, some sleep deprived animals may die faster than others. While there is no definite conclusion, most experts agree that the need for sleep varies from species to species, as does the function of sleep, and there must be more research to substantiate some of the more popular theories about animal sleep.
One of the most relevant studies in measuring REM needs in animals is the research done comparing evolutionary influences on sleep patterns and analyzing how animals sleep. By understanding how does sleep work in animals, one can see how tracking back through evolution revealed animal populations, which were under high stress related to the fear of being preyed on, needed less sleep than those that were not at risk from predators. These ecological studies seem to be relevant and correlate with the theory that the intelligence of the animal and REM sleep needs is not as direct as other causes such as evolutionary and ecological reasons.
