We examine the effect of diel light:dark cycles (12:12 LD) on the production of free fatty acids (FFAs) in Synechocystis sp. PCC 6803. Our results show that FFA titers in FFA producing strains of Synechocystis were significantly reduced in 12:12 LD compared to continuous light. On the transcriptional level, we saw no significant changes of the fatty acid biosynthesis pathway as a result of increased FFA production. In addition, we describe our efforts in increasing FFA production by introducing 'tesA from Escherichia coli. Despite evidence of 'tesA mRNA transcript abundance, no measurable TesA enzymatic activity was observed in our Synechocystis mutants. Overall, our work demonstrates the importance of considering the effects of diel light:dark cycles in the metabolic engineering of cyanobacteria.