Abstract: Providing light during incubation has been shown to hatchability and post hatch development; however, the optimal timing of this light is still not known. To determine if there is an effect of exposing embryos to light for the first 18 d of incubation or the entire 21 d of incubation, we incubated broiler chicken eggs (N = 3096) for either 18 d (18DL), 21 d (21DL) or not at all (DARK). Embryo mortality, chick weight, proportion of cull or dirty chicks, or those with leg or other abnormalities was not affected by incubation conditions (p>0.05). The DARK broilers had a lower hatch rate, a lower proportion of unhealed navels and no defect chicks than both the 18DL and the 21DL (p<0.05) broilers. There was no effect of incubation condition (p>0.05) on 45 d weight gain or 45 d feed conversion. The DARK broilers vocalized more during isolation and had longer latency to right during tonic immobility than both the 18DL and the 21DL (p<0.05) broilers. The DARK broilers had higher composite asymmetry scores and corticosterone concentrations than both the 18DL and the 21DL (p<0.05) broilers. There were no differences observed between the lighting treatments in any measure (p>0.05). Hatchability, chick quality, fear response and stress susceptibility were all improved with lighted incubation irrespective if the lighting occurred for 18 or 21 days. Providing light during the first 18 d of incubation can improve production and bird welfare.