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Timing and circadian effects in fatigue models

One of the most important parts of a fatigue risk model is timing: when work happens, not just how long it lasts. This page explains the timing (circadian) part of fatigue modelling in plain English.

It is not medical advice and does not set fixed cut-off times. Night and early work are not “prohibited” — the point is to understand and manage the added exposure.

Time of day and the body clock

Section titled “Time of day and the body clock”

Human alertness follows a daily rhythm driven by the body clock (circadian rhythm). Alertness tends to fall during the biological night and reach a low in the early hours of the morning, then recover through the day.

Fatigue models reflect this: the same task can carry a different modelled fatigue estimate depending on the time of day it is performed.

Biological night, night shifts and early starts

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  • Night shifts place work across the circadian low, when the body is primed for sleep. See night shift fatigue.
  • Early starts require waking before the natural rise in alertness and often compress the sleep beforehand. See early starts and fatigue.
  • HSE’s calculator guidance notes that fatigue and risk peak at slightly different times — risk tends to peak close to midnight, while modelled sleepiness tends to peak some hours later in the early morning. This is one reason the model produces both a Fatigue Index and a Risk Index.

Circadian disruption

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Working against the body clock — especially rotating between days and nights — can cause circadian disruption, where the timing of the internal clock and the work schedule are out of step. Models capture the general effect of this at a population level; they cannot know how well a specific individual has adapted.

RR446 considered whether permanent night workers would need a separate index and concluded that suitability depends on the degree of adaptation to the night shift — a reminder that individual factors vary.

Why two equal-length duties can differ

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Because timing matters, two duties of the same length can carry different modelled fatigue exposure. For example:

  • A duty starting mid-morning versus one starting at 04:00
  • A day shift versus a night shift of identical duration
  • An early start after adequate sleep versus the same start after a late finish the day before

The timing component is why a fatigue model can distinguish these, whereas simply counting hours cannot.

How timing interacts with sleep and travel

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Timing does not act alone:

  • Sleep opportunity — early starts and night finishes can cut into the main sleep period, so the timing of a duty affects how much sleep is realistically available.
  • Travel — a long journey before an early start extends wakefulness and further reduces sleep. See door-to-door time and duty, travel, and commute.
  • Cumulative load — repeated early or night duties compound the effect. See cumulative fatigue in models.

A model estimates the combined effect, but the result should be interpreted with competent judgement and checked against how tiring workers actually find the pattern.

What the timing component does not do

Section titled “What the timing component does not do”
  • It does not set a universal safe start time or a fixed cut-off — there are no fixed universal safe shift times.
  • It does not measure real-time alertness or determine fitness for duty.
  • It does not mean night or early work is never acceptable — it means the added exposure should be assessed and managed.

See limitations of fatigue models for the wider caveats.

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References

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