Circadian Influences on Pre- and Post-Admission Events in Patients with Acute Aneurysmal Subarachnoid Hemorrhage

Background Aneurysmal subarachnoid hemorrhage (aSAH) is a life-threatening condition with high morbidity and mortality. Emerging evidence suggests that circadian rhythms may influence both the timing of aneurysmal rupture and physiological processes during hospitalization. Understanding these temporal patterns may provide insight into underlying pathophysiological mechanisms and inform the timing of interventions. Objective To review and synthesize current literature on the circadian timing of events before and after hospital admission in aSAH patients, including rupture onset, circadian biomarkers, therapeutic timing, and clinical outcomes. Methods We conducted a comprehensive review of studies examining diurnal and circadian patterns in aSAH, including the timing of symptom onset, fluctuations in physiological biomarkers (melatonin, cortisol, temperature), gene expression, and the timing of surgical or endovascular interventions. Subgroup analyses were reviewed for demographic and clinical modifiers such as age, sex, comorbidities, and substance use. Results Many studies report a primary peak in aSAH onset during the morning hours, with a secondary evening peak and a consistent nighttime nadir, suggesting a bimodal distribution. Subgroup analyses reveal potential influences of age, sex, and comorbidities on circadian patterns. Aneurysmal SAH rupture was associated with disruption of circadian biomarkers, including blunted melatonin and cortisol rhythms, altered thermoregulation, and suppressed expression of the circadian gene PER2. Timing of aneurysm repair may influence outcomes, with some studies suggesting worse prognosis for nighttime interventions, though findings were confounded by treatment delays and illness severity. Conclusion Circadian rhythms appear to play a significant role in the pathophysiology of aSAH, influencing not only rupture timing but also downstream clinical processes and outcomes. Disruption of biological rhythms post-SAH is common and may carry prognostic value. Further research is needed to standardize timing methodology, elucidate underlying mechanisms, and evaluate chronobiology-informed interventions to improve outcomes in aSAH patients.