Purpose: In this study, the relationship between the plasma levels of melatonin and intercellular adhesion molecule-1 (ICAM-1), which plays role in several intercellular interactions including inflammatory and immune responses, and early neurocognitive functions associated with ischaemia-reperfusion injury during open heart surgery is examined.Methods: Forty patients who were to undergo elective coronary artery bypass grafting (CABG) were divided into two groups, those who underwent their operations at 8 AM (group I; n = 20) and those who underwent their operations at 1 PM (group II; n = 20). Blood samples were collected prior to surgery (S1), when the aortic cross clamp was removed (S2) and 4 (S3) and 24 h after the surgery (S4). Neuropsychiatric assessment was conducted one day before and seven days after surgery.Results: Melatonin levels measured during and after surgery were also significantly higher in Group 1. ICAM-1 levels were significantly lower in Group 1 at S2 and S3. Significant deterioration was observed in postoperative neurocognitive function compared with preoperative functions in Group 2 more than Group 1.Conclusion: We hypothesise that the greater preservation of neurocognitive functions in the morning patients is associated with elevated melatonin levels, which reduce the damage from ischaemia-reperfusion injury.