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 or nocturnal. The cathemeral activity pattern was first described by Ian Tattersall in 1979, describing the common brown lemur.
Many animals do not fit the traditional definitions by being strictly nocturnal, diurnal, or crepuscular, often due to factors that include the availability of food, predation pressure, and variable ambient temperature. As a result, many species, particularly among primates, may be classified as cathemeral.
Alternative patterns of cathemeral activity have been observed in specific lemurs. Seasonal cathemerality has been described for species Eulemur mongoz (Mongoose lemur) as activity shifting from being predominantly diurnal to being predominantly nocturnal over a yearly cycle. E. fulvus fulvus (Common brown lemur) has been observed seasonally shifting from diurnal activity to cathemerality.