Background: Sleep disturbance is an important medical problem in patients with end-stage renal disease. It might be related to the disruption of the body's circadian clock since nocturnal levels of its key biomarker melatonin are markedly reduced. We aimed at investigating whether a change in renal function due to kidney transplantation or donation would modify sleep, melatonin levels, circadian rhythmicity, and quality of life in kidney transplant recipients (KTR) and living donors (LD). Methods: In KTR, we assessed saliva mel-atonin concentrations, sleep quality and daytime sleepiness prior to and at 2 weeks and 3 months after transplantation. In LD, we assessed these parameters prior to and at 3 months after donation. We additionally assessed 24-hour core body temperature (cBT), 24-hour blood pressure profile, and quality of life (QoL) prior to and 3 months after transplantation. Results: Twenty-three KTR and 23 LD completed the study. Regarding sleep, the amount of nighttime awake minutes tended to be reduced in recipients after transplantation (p = 0.05). Nocturnal melatonin concentrations did not change with transplantation or donation. Blood pressure dipping profile and the two circadian markers dim-light melatonin onset and time of core body temperature minimum did not change. Nevertheless, KTR reported that daytime sleepiness and QoL had improved. Conclusion: Objectively nocturnal sleep quality marginally improved after transplantation. Subjectively patients reported improved QoL and daytime sleepiness scores. Changes in renal function were not associated with modified melatonin secretion or circadian rhyth-micity.