This paper is focused on optimizing the amount of light duty cycle of a photo-bioreactor operating in batch mode. The mathematical model used is confined to one dissolved substrate, one biomass (algae), one internal cell quota, and the irradiance for photo-acclimated culture. The model has been previously published and validated with experimental data. The following optimization problem is studied: minimize the effluent substrate concentration subject to: maximum and minimum amount of light to be used, the time of the light/dark illumination and the total time of the batch experiment. Analytical solution for this optimization problem seems difficult to obtain. However, numerical results obtained from simulations show that it is possible to find solutions which satisfy the problem requirements.