Experimental studies on the thermal biology of organisms have become crucial to investigate the impact of climate warming. However, most laboratory studies are carried out under constant temperatures and assume a negligible effect from daily fluctuating temperatures. We tested this assumption on multiple fitness traits of the moth Spodoptera littoralis, and a literature review on insects complements this study. Tests on S. littoralis focused on its optimal and maximal critical temperatures by comparing constant and daily fluctuating temperatures (± 5°C) at mean temperatures of 25, 29 and 33°C. The nine fitness parameters investigated were influenced by mean temperature. The overall effect was a maximal multiplication rate at 29°C and a marked decrease under the fluctuating regime at 33°C. Effects of fluctuating temperatures differed between mean temperatures. Developmental and larval survival rates at 33°C were lower under the fluctuating thermal regime than under a constant temperature. Our literature review also illustrates that ignoring daily fluctuations based on constant temperatures commonly leads to overestimate fitness traits at high temperatures. Overlooking the experimental bias associated with constant temperatures minimizes the expected impact of climate warming on fitness traits.