“Time is brain” is one of the most influential principles in the management of acute ischemic stroke. It has appropriately driven the development of rapid stroke pathways, prehospital triage, streamlined imaging protocols, and efficient reperfusion systems. Yet, for one important group of patients, the clock presents a fundamental challenge: those who awaken from sleep with neurological deficits have a known last-known-well time, but an unknown time of stroke onset.
For decades, this uncertainty largely translated into therapeutic exclusion. A patient who went to sleep neurologically intact and awoke with aphasia, hemiparesis, or visual loss was often considered ineligible for intravenous thrombolysis because the stroke could have occurred at any point during sleep. But this approach rests on an important assumption—that chronological time accurately reflects the biological age of the infarct in every patient.
It does not.
The management of wake-up stroke has therefore become an important example of a broader transformation in acute stroke medicine: the movement from a predominantly time-based paradigm toward a tissue-informed paradigm.
From the clock to the tissue
The central question in wake-up stroke should not simply be, “When did the stroke occur?” It should also be, “How much brain is already irreversibly injured, and how much remains potentially salvageable?” (Figure 1).
The WAKE-UP trial provided a landmark demonstration of this principle. Patients with an unknown time of onset were selected using magnetic resonance imaging, specifically the presence of an acute ischemic lesion on diffusion-weighted imaging (DWI) without marked corresponding hyperintensity on fluid-attenuated inversion recovery (FLAIR). This DWI-FLAIR mismatch was used as an imaging marker suggesting a relatively recent stroke. In the trial, intravenous alteplase resulted in a significantly greater proportion of patients achieving an excellent functional outcome at 90 days compared with placebo.[1]
The importance of WAKE-UP extended beyond its numerical results. It challenged the assumption that time since last known well and biological time since infarction are interchangeable.
Consider two patients who both arrive at the emergency department at 8 a.m. One may have developed a rapidly progressive infarction several hours earlier with a large established core. Another may have developed the stroke shortly before awakening and still have substantial viable tissue. The clock shows the same time. The biology does not.
Perfusion imaging: identifying the mismatch
The tissue-based concept was subsequently strengthened by perfusion-selected thrombolysis, as illustrated in Figure 1B. In EXTEND, patients presenting between 4.5 and 9 hours after symptom onset, including patients with wake-up stroke, were selected using automated perfusion imaging to identify a small ischemic core with potentially salvageable tissue. Alteplase improved the likelihood of an excellent functional outcome compared with placebo, although the risk of symptomatic intracranial hemorrhage was higher.[2] An individual-patient-data meta-analysis incorporating EPITHET, ECASS-4, and EXTEND subsequently demonstrated that perfusion-selected patients treated in the 4.5–9-hour window had better functional outcomes with thrombolysis than with control treatment.[3]
These studies collectively support a simple but important concept:
A late presentation is not necessarily equivalent to a late biological state.
This distinction is particularly relevant in wake-up stroke, where the conventional onset clock is inherently unreliable.
The clock still matters
However, the emergence of tissue-based selection should not be misinterpreted as the death of the clock.
Time remains critically important. For patients who are clearly within the standard intravenous thrombolysis window and otherwise eligible, treatment should be initiated as rapidly as possible. The 2026 American Heart Association/American Stroke Association guideline specifically emphasizes that eligible patients within 4.5 hours should receive rapid thrombolysis without unnecessary delays for advanced imaging.[4]
This distinction is crucial.
Tissue-based selection should not become imaging-based delay.
Advanced imaging is most valuable when the conventional time window cannot adequately answer the treatment question—for example, in wake-up stroke or other patients presenting beyond the standard window.
Thus, the appropriate paradigm is not:
“Time no longer matters.”
It is:
“Time remains critical, but tissue can provide additional information when time is uncertain.”
What has changed in 2026?
The expanding evidence base is now reflected in the 2026 AHA/ASA guideline for the early management of acute ischemic stroke. The guideline incorporates imaging-selected intravenous thrombolysis for appropriately selected patients with unknown or extended time of onset, including approaches based on DWI-FLAIR and perfusion mismatch.[4]
For patients with wake-up stroke or unclear onset beyond 4.5 hours from last known well, mismatch-based selection can therefore identify patients who may still benefit from intravenous thrombolysis. The guideline also recognizes perfusion-selected treatment in appropriate patients who awaken with stroke symptoms within 9 hours from the midpoint of sleep or who present 4.5–9 hours from last known well.[4] This represents an important philosophical change in acute stroke care. Unknown onset should no longer automatically mean unknown opportunity.
Wake-up stroke is not synonymous with “too late”
The term wake-up stroke can unintentionally reinforce a misleading clinical intuition. It describes how the stroke was discovered, not how much brain has been lost. Figure 1C further illustrates this distinction.
A patient waking with a disabling neurological deficit should therefore not be dismissed simply because the onset cannot be witnessed. Instead, the patient should undergo urgent assessment for hemorrhage, large-vessel occlusion, infarct burden, and—where appropriate—tissue viability.
The question should become:
Is the deficit disabling?
Is there a large established infarct?
Is potentially salvageable tissue present?
Is the patient a candidate for thrombolysis and/or thrombectomy?
This is a fundamentally different approach from simply asking whether the patient is “within 4.5 hours.”
Thrombolysis and thrombectomy must be considered together
Wake-up stroke should also never be viewed exclusively through the lens of thrombolysis. A patient presenting with a disabling deficit may have a large-vessel occlusion and may be an appropriate candidate for mechanical thrombectomy based on clinical and imaging criteria.[5-7]
Therefore, vascular imaging and assessment for large-vessel occlusion should occur rapidly and should not be unnecessarily delayed by the uncertainty surrounding symptom onset. The modern stroke pathway is increasingly about identifying the right reperfusion strategy rather than forcing every patient into a single chronological window.
The challenge for real-world practice
The greatest challenge may no longer be whether the evidence exists, but whether healthcare systems can deliver it.
Tissue-based selection requires appropriate imaging, rapid interpretation, validated automated processing where applicable, trained stroke teams, and established reperfusion pathways. These resources are not uniformly available, particularly in resource-constrained settings.
This creates an important implementation gap. The evidence base is moving toward individualized treatment, while many stroke systems remain organized around rigid time thresholds.
The answer should not be to abandon tissue-based selection. Rather, stroke networks need pragmatic pathways that use available imaging efficiently, minimize treatment delays, and rapidly transfer appropriate patients to centers capable of advanced reperfusion therapy.
A new question for the emergency department
The phrase “time is brain” remains entirely relevant. Every unnecessary minute before reperfusion can cost functional brain tissue.
But time is a surrogate—not the brain itself.
Patients differ in collateral circulation, infarct progression, vascular anatomy, and susceptibility to irreversible injury. Some are rapid progressors; others retain viable tissue despite apparently prolonged or unknown onset times.
The future of acute stroke treatment therefore lies not in abandoning the clock, but in placing it in context with tissue, vessel status, clinical severity, and individual biological variability.
For the patient who wakes with a disabling stroke, an unknown onset should prompt urgency, not therapeutic nihilism.
Perhaps the most useful question is no longer simply:
“Is it too late to lyse?”
but rather:
“Is there still brain worth saving—and can we identify it quickly enough to save it?”
That is the direction in which modern stroke medicine is moving: from treating the clock alone to treating the patient in front of us, guided by the biology of the brain that remains.
Declarations
Contributors:
MN conceived the idea, drafted, and critically revised the article.
Funding:
The author have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests:
None declared.
Provenance and peer review:
Not commissioned; internally peer reviewed.
Patient consent for publication:
Not applicable.
References
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- 2.Ma H, Campbell BCV, Parsons MW, Churilov L, Levi CR, Hsu C, et al; EXTEND Investigators. Thrombolysis guided by perfusion imaging up to 9 hours after onset of stroke. N Engl J Med. 2019;380(19):1795-1803. doi: 10.1056/NEJMoa1813046.
- 3.Campbell BCV, Ma H, Ringleb PA, Parsons MW, Churilov L, Bendszus M, et al; EXTEND, ECASS-4, and EPITHET Investigators. Extending thrombolysis to 4.5-9 h and wake-up stroke using perfusion imaging: a systematic review and meta-analysis of individual patient data. Lancet. 2019;394(10193):139-147. doi: 10.1016/S0140-6736(19)31053-0.
- 4.Prabhakaran S, Gonzalez NR, Zachrison KS, Adeoye O, Alexandrov AW, Ansari SA, et al. 2026 Guideline for the early management of patients with acute ischemic stroke: a guideline from the American Heart Association/American Stroke Association. Stroke. 2026;57(8):e316-e436. doi: 10.1161/STR.0000000000000513.
- 5.Nogueira RG, Jadhav AP, Haussen DC, Bonafe A, Budzik RF, Bhuva P, et al; DAWN Trial Investigators. Thrombectomy 6 to 24 hours after stroke with a mismatch between deficit and infarct. N Engl J Med. 2018;378(1):11-21. doi: 10.1056/NEJMoa1706442.
- 6.Albers GW, Marks MP, Kemp S, Christensen S, Tsai JP, Ortega-Gutierrez S, et al; DEFUSE 3 Investigators. Thrombectomy for stroke at 6 to 16 hours with selection by perfusion imaging. N Engl J Med. 2018;378(8):708-718. doi: 10.1056/NEJMoa1713973.
- 7.Goyal M, Menon BK, van Zwam WH, Dippel DWJ, Mitchell PJ, Demchuk AM, et al; HERMES Collaborators. Endovascular thrombectomy after large-vessel ischaemic stroke: a meta-analysis of individual patient data from five randomised trials. Lancet. 2016;387(10029):1723-1731. doi: 10.1016/S0140-6736(16)00163-4.