Same Pattern of Circadian Variation According to the Season in the Timing of Ischemic Stroke Onset: Preliminary Report

Background and Objective Stroke occurrence shows a chronobiological variation, which is considered to be related with waking in the morning. We hypothesized that its seasonal difference would also exist because individual life activity including sleep-wake pattern can be influenced by interseasonal variability. The objective of this study was to investigate the seasonal difference of circadian variation in the timing of onset in patients with acute ischemic stroke. Methods We studied 1486 patients with acute ischemic stroke. Stroke onset time was defined as the earliest time the patient or a witness noted definite neurological symptoms. The patients with clear onset stroke were enrolled. Frequency of onset was analyzed for four 6-hour and twelve 2-hour intervals from 0 to 24 hour per day. Results The clear onset time was known in 968 patients (65.1%). Mean age of the patients was 67.63 ± 12.65 years and 589 patients (60.9%) were male. Stroke occurred in spring (26.5%), summer (27.2%), fall (24.1%), and winter (22.3%). In all cases, ischemic stroke showed a significant circadian variation in time of onset (p < 0.001). High peak period was between 6:01 AM to 12:00 PM (37%) with the same pattern in each season. Conclusions Seasonal difference was not significant despite circadian variation in time of onset of ischemic stroke. Thus, exogenous factors such as environmental factors and life styles may have less influence on inter-seasonal variability of circadian rhythms, which are related with chronobiological factors of stroke onset in this regional population.