Whether a lizard is nocturnal or diurnal really just depends on the species. Some lizards are nocturnal where others are diurnal. For example, bearded dragons are diurnal which means that they awaken during the day whereas lizards such as the leopard gecko prefer to be awake at night and sleep more during the day, making them nocturnal.
Furthermore, diurnal lizards follow a human sleep pattern and are active and eat during the day, while sleeping at night. Nocturnal lizards are active during the night and will sleep during the day time and hunt for its food all night.