Damage in ischemic stroke is more severe in the evening or at night.
The severity of ischemic stroke depends on the time of day it occurs: in the evening and at night, brain damage is more pronounced. This conclusion was reached by researchers who studied the role of circadian rhythms of the immune system in the development of stroke. The work was published in the journal Circulation Research (CR).
The scientists showed that the key factor is the daily activity of neutrophils — the most numerous cells of the innate immune system. In experiments on mice, strokes that occurred during the 'inactive phase' (analogous to evening and nighttime hours in humans) were accompanied by larger areas of damage and more severe neurological impairments. During this time, neutrophils transitioned into an aggressive pro-inflammatory state and actively formed extracellular traps (NETs) — structures made of DNA and proteins that can clog microvessels.
These traps disrupted microcirculation in the brain and worsened collateral blood flow — the system of bypass vessels that temporarily supplies blood to ischemic areas. When researchers suppressed NET formation or disrupted the neutrophils' 'biological clocks,' the differences between daytime and nighttime strokes disappeared, and blood flow was restored.
Analysis of data from more than 500 patients with ischemic stroke confirmed these observations: in the evening and nighttime hours, the levels of neutrophil activity markers and NETs increased in the blood, which coincided with worse collateral circulation and less favorable clinical outcomes.
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