A cathemeral organism is one that has sporadic and random intervals of activity during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are performed. It has been defined as follows: "The activity of an organism may be regarded as cathemeral when it is distributed approximately evenly throughout the 24 h of the daily cycle, or when significant amounts of activity, particularly feeding and/or traveling, occur within both, the light and dark portions of that cycle."

Until the late 1970s activity patterns of primates were only categorized as diurnal
Diurnal animal
Diurnality is a plant or animal behavior characterized by activity during the day and sleeping at night.-In animals:Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects, reptiles and birds...

 or nocturnal
Nocturnal animal
Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The common adjective is "nocturnal"....

. The cathemeral activity pattern was first described by Ian Tattersall
Ian Tattersall
Ian Tattersall is a paleoanthropologist and a curator at the American Museum of Natural History. Tattersall received his PhD from Yale University in 1971. In addition to human evolution, he has worked extensively with lemurs. He is working with The Templeton Foundation.-Selected publications:* The...

 in 1979, describing the common brown lemur
Common Brown Lemur
The common brown lemur , or brown lemur, is a species of lemur in the Lemuridae family. It is found in Madagascar and Mayotte.-Range:...

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