Most people believe that leopards are nocturnal but it is not entirely true. In areas where they are hunted extensively or persecuted by people, they are nocturnal in order to avoid people. Leopards are less nocturnal and more terrestrial where there are less tigers and lions.
In the Chitwan NP in southern Nepal, leopards living along the “park-village interface were essentially nocturnal”. This was to avoid the people living in the villages. Leopards avoid tigers. For example in Chitwan NP leopards rarely use roads to travel on whereas tigers use them often (Mel Sunquist 1981 – The Social organisation of tigers in Royal Chitwan National Park).
In the Kalahari, leopards mainly move about at night but often rest during the beginning of the night and again just before dawn (Bothma J du P and EAN Le Riche 1986 Prey preference and hunting efficiency of Kalahari Desert leopards).
In the Judaean desert of southern Israel leopards rarely hunt at night. They might hunt at night when hunting porcupines or domestic cats around homes. In this area the leopard mainly preys on hyraxes and ibex. Both these animals are diurnal meaning that they are active during the day and night so the leopard is also diurnal in this place (Ilany G 1981. The leopard and the Judean desert).
In Cape province, South Africa, leopards are mostly diurnal. They appear to be more active in late morning and again in late afternoon or early evening. They rest from midnight until sunrise. In the Cape mountains leopards prey extensively on rock hyrax. These animals are diurnal and therefore so is the leopard. The same goes for small antelopes (Norton PM and SR Henley 1987 Home range and movements of male leopards in the Cedarberg Wilderness Area and same reserchers but including G Avery 1986 in Prey of leopards in four mountainous areas of the south-western Cape Province).
In the Huai Kha Khaeng Sanctuary, Thailand, both a male and female leopard were radio tracked and found to be moving during both the daytime and at night. Similarly leopards in the Kaeng Krachan National Park, Thailand, were also active during the daytime and at night. However, activity is “strikingly different from most observations of leopards” (Rabinowitz A 1989 The density and behavior of large cats in a dry tropical forest mosaic in the Huai Kha Khaeng Sanctuary, Thailand and Grassman LI Jr 1999 in Ecology and behavior of the Indochinese leopard in Kaeng Krachan National Park, Thailand).
The conclusion, on my research, is that leopards tend to be nocturnal in order to avoid predators such as humans, lions and tigers but are not uniquely nocturnal. It depends upon the circumstances and the availability of prey. As you can see, leopards are active in the daytime when prey is also active at that time.
P.S. These studies were carried out some time ago. Patterns of behavior may have changed in these areas since but the fundamental lesson remains valid.