Unlike other species, when tomato plants (Solanum lycopersicum L.) are deprived of at least 8 h of darkness per day, they develop a potentially lethal injury. In an effort to understand why continuous light (CL) is injurious to tomato, we tested five factors, which potentially could be responsible for triggering the injury in CL-grown tomato: (i) differences in the light spectral distribution between sunlight and artificial light, (ii) continuous light signalling, (iii) continuous supply of light for photosynthesis, (iv) continuous photo-oxidative pressure and (v) circadian asynchrony – a mismatch between the internal circadian clock frequency and the external light/dark cycles. Our results strongly suggest that continuous-light-induced injury does not result from the unnatural spectral distribution of artificial light nor from the continuity of light per se. Instead, circadian asynchrony seems to be the main factor inducing the CL-induced injury, but the mechanism is not by the earlier hypothesised circadian pattern in sensitivity for photoinhibition. Here, however, we show for the first time diurnal fluctuations in sensitivity to photoinhibition during normal photoperiods. Similarly, we also report for the first time diurnal and circadian rhythms in the maximum quantum efficiency of PSII (Fv/Fm) and the parameter F0.