ABSTRACT: The influence of different photoperiod cycles (3L : 3D, 6L : 6D, 12L : 12D, and 24L : 24D) on the metabolic rate and energy loss of either fed or unfed young tilapia Oreochromis niloticus (bodyweight 8.6–9.5 g) was investigated at 28°C. A computer-operated respirometer with a closed tank was used to prevent water from evaporating into the air or condensing from the air. The photoperiods acted as strong Zeitgeber (i.e., cue or synchronizer) during the experiments with either fed or unfed fish. A photoperiod-mediated metabolic cycle was demonstrated in fed and unfed fish in which oxygen consumption was higher during the light phase compared with during the dark phase for all photoperiods. Mean oxygen consumption during the 3L : 3D, 6L : 6D, 12L : 12D, and 24L : 24D periods for fed and unfed fish were 685.06 mg/kg per h and 299.33 mg/kg per h; 658.52 mg/kg per h and 284.80 mg/kg per h; 591.09 mg/kg per h and 249.62 mg/kg per h; and 500.64 mg/kg per h and 239.14 mg/kg per h, respectively. The highest post-prandial increase in energy loss was recorded during the 3L : 3D period (145.88 kJ/kg per day), followed by 141.19 kJ/kg per day during the 6L : 6D, 128.70 kJ/kg per day during the 12L : 12D, and 99.92 kJ/kg per day during the 24L : 24D periods. The results suggest that higher energy conservation would be achieved if fish are exposed to longer rather than shorter photoperiod cycles.