The pervasive occurrence of circadian clocks throughout the living world underlines their adaptive value. Nonetheless, there is surprisingly little evidence for a negative impact, on any animal species, of a constant discrepancy between the environmental and endogenous periods. Male Drosophila melanogaster per mutants with altered circadian periods were compared to the wild type in two different LD schedules. Life span was used as a global index of physiological adaptation. The life span of the mutants was significantly reduced by up to 15% for the flies whose period differs most from that of the wild type. A reduction was observed even when flies were kept in an LD schedule fitting a mutant period. The LD schedule made no significant difference on its own, but the authors found evidence for an interaction between genotype and LD schedule in determining life span. These results are consistent with the importance of the circadian clock in maintaining internal temporal order independent of environmental cycles. Nonetheless, a large difference between the environmental and endogenous periods has a measurable impact.