Traditionally, the activity patterns of primates have been classified as nocturnal and diurnal, with the former recognized as the ancestral trait. Since cathemerality, i.e., active both day and night, was discovered in Eulemur (Lemuridae) in the 1960s, the evolutionary origin and mechanism of cathemerality have been explored as a key to understanding diurnalization in primates. To understand cathemerality in lemurs, this article reviews current knowledge and outlines future issues. Although several theories have hypothesized that cathemerality is an evolutionary disequilibrium condition, as a consequence of recent, incomplete diurnalization, the current analyses of phylogenetic history and eye morphology conclude that it is an adaptive strategy that originated in the common ancestor of Lemuridae around 46-20 MYA. In Madagascar, the days are generally long during the rainy season and short during the dry season. Cathemeral lemurs often increase their diurnal activities in the rainy season and nocturnal activities in the dry season. Chronobiological approaches that have explored the proximate mechanisms have clarified that the light-dark cycle controls the daily activity rhythms and the day-length cause seasonal shift of activity patterns as zeitgebers. In addition, moonlight has a masking effect that facilitates nocturnal activities. Ecological/ethological approaches have examined four hypotheses, as ultimate mechanisms: avoidance of predation risk, relief of interspecific competition, thermoregulation, and extension of feeding activities. However, there is evidence supporting and countering all four hypotheses. Therefore, cathemerality cannot be defined as an adaptive consequence of any single factor. Consequently, cathemerality is recognized as a flexible strategy for dealing with several factors in the harsh, unpredictable Madagascar environment. In the future, researchers need to examine flexible activities in response to other factors, such as habitat disturbance caused by humans, to explain complex mechanisms caused by compound factors, and to compare the activities of diurnal lemurs using ecological and physiological approaches.