Bromundt et al ^{Reference Bromundt, Köster, Georgiev-Kill, Opwis, Wirz-Justice and Stoppe1} need to be commended for using sleep measures, an often neglected component of psychiatric disorders, to investigate the role of sleep and circadian rhythms in influencing cognition in patients with schizophrenia.

However, there is one major omission in their method: the time of day that the neuropsychological tests were conducted and whether these were consistent across groups. Previous research has shown that neurobehavioral functioning is correlated to the core body temperature and timing of melatonin secretion. ^{Reference Wyatt, Ritz-De Cecco, Czeisler and Dijk2} If the timing of the tests was inconsistent across the patients, it could account for at least some of the differences seen between the groups.

Also, in this study, the group with low amplitude had a delayed onset of melatonin secretion. This could be seen in patients with delayed sleep phase syndrome. ^{Reference Wyatt, Stepanski and Kirkby3} Potentially, conducting these tests in the morning in patients with delayed sleep phase syndrome could result in poor performance compared with patients with normally entrained sleep. Future research would need to account for these differences and tailor testing times to minimise chances of bias.