The improvement of photosynthetic efficiency in a 100 m2 raceway reactor by enhancement of light regime to which the cells are exposed is here reported. From Computational Fluid Dynamics it was calculated that the light exposure times ranged from 0.4 to 3.6 s while the exposure times to darkness were much longer, from 6 to 21 s. It was demonstrated that these times are too long for light integration, the cells fully adapting to local irradiances. This phenomenon was validated in the real outdoor raceway at different seasons. Simulations allows to confirm that if total light integration is achieved biomass productivity can increase up to 40 g/m2·day compared to 29 g/m2·day obtained considering local adaptation, which is close to the experimental value of 25 g/m2·day. This paper provides clear evidence of microalgae cell adaptation to local irradiance because of the unfavourable cell movement pattern in raceway reactors.