By 4pm, Your Anaesthesia Risk Is Four Times What It Was at 9am
The time of your surgery matters more than most people realise. By late afternoon, anaesthesia-related adverse events are four times more likely than first thing in the morning.
The Short Answer: Research shows anaesthesia adverse events are four times more likely at 4pm than at 9am. Fatigue, circadian rhythm, and cumulative cognitive load all contribute to the increased risk as the day progresses.
If you have any say in when your surgery is scheduled, the evidence points firmly in one direction: choose the morning.
The Evidence
Wright and colleagues analysed anaesthetic adverse events by time of day and found the predicted probability quadrupled from 1.0% at 9am to 4.2% by 4pm. A fourfold increase over seven hours.
A 2025 retrospective cohort study of 14,394 high-risk patients confirmed the pattern with even starker numbers. Surgeries starting between 3pm and 6pm carried 33% higher one-year mortality and 32% more complications compared with morning cases.
Why Afternoon Surgery Carries More Risk
Two factors combine. First, circadian biology. Human alertness, reaction time, and cognitive function naturally dip in the afternoon. The post-lunch circadian trough is well documented across professions. Air traffic controllers, long-haul drivers, and surgeons all experience the same decline.
Second, cumulative fatigue. By the afternoon, surgical teams have been making high-stakes decisions for hours. Concentration erodes. Vigilance drops. Fine motor skills decline incrementally but measurably.
The effects are not dramatic enough for anyone to notice in real time. Nobody feels four times less competent at 4pm. But the data, aggregated across thousands of cases, shows the pattern with uncomfortable clarity.
What Can Be Done
Schedule-level interventions help: prioritising complex cases in the morning, building in team breaks, rotating high-focus roles. But individual-level factors matter too. How well rested the team is. How comfortable they are physically. How many small, accumulating sources of distraction they face across a long day.
Surgeon wellbeing is not a soft concern. It is a measurable clinical variable with a demonstrated impact on patient outcomes. Thermal comfort, adequate hydration, and attire that does not add unnecessary physical stress all contribute to sustained performance.
A scrub cap that wicks moisture and breathes properly is part of maintaining the conditions under which surgical teams perform at their best, particularly during those critical afternoon hours.
Source: See references cited in the article above.
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