Most species experience some amount of temperature variation at daily and seasonal timescales. Variations in temperature are expected to shape key biological traits of ectotherms, but also the breadth of phenotypes expressed within a population. Here, we combine meta-analytical and experimental data to uncover how daily temperature variation impacts phenotypic means and variation of ectothermic animals. We hypothesised that variation around warm temperatures would decrease growth, locomotion and reproductive success but increase phenotypic variation in these traits. We tested this hypothesis by first assembling the most comprehensive database to date, spanning 165 ectothermic species, and performing meta-analyses. Second, using black bream (Acanthopagrus butcheri) as an experimental model, we assessed the upper thermal limits of fish following acclimation to constant or variable daily temperatures, and compared these data to the thermal limits of field-acclimated fish. Our meta-analysis revealed that daily temperature variation increases upper thermal limits across ectotherms, but reproductive traits were compromised. Surprisingly, phenotypic variation did not increase in response to daily temperature variations. Experimental data showed that upper thermal limits of field-acclimated fish does not mirror that of fish acclimated to a constant temperature under lab conditions. Taken together, these findings demonstrate the importance of considering how daily temperature variation impacts key biological traits. Experiments that expose ectotherms to constant temperatures can over-simplify physiological responses to temperature variation that could misguide conservation and management actions.