Skip to content
fatiguemanagement.co.uk

HSE RR446 and the fatigue risk index

HSE Research Report RR446 — The development of a fatigue / risk index for shiftworkers (2006) — is the key public research source behind the UK fatigue risk index. It was prepared for the Health and Safety Executive by QinetiQ and Simon Folkard Associates.

This page explains what RR446 is and how to interpret it responsibly. It is not legal advice, and RR446 is not a legal compliance threshold.

What RR446 is

Section titled “What RR446 is”

RR446 is a research report. It describes the work carried out to revise and update the earlier HSE Fatigue Index, drawing on research into sleep, the body clock, shift length, breaks, and recovery across a sequence of shifts.

Importantly, RR446 states that its contents — including any opinions and conclusions — are those of the authors and do not necessarily reflect HSE policy. It is a scientific basis for a biomathematical fatigue model, not a rulebook.

Why it was developed

Section titled “Why it was developed”

The report set out to improve on the previous fatigue index by incorporating more recent evidence on:

  • Cumulative fatigue across a sequence of shifts
  • Time-of-day (circadian) effects
  • Shift length
  • The effect of breaks
  • Recovery between shifts

It also reviewed trends in risk related to shift work, which led to a significant outcome: two separate indices rather than one.

What the Fatigue Index and Risk Index are

Section titled “What the Fatigue Index and Risk Index are”

RR446 produced two related outputs, calculated from the same roster information but answering different questions:

  • Fatigue Index (FI) — a modelled estimate of the probability that a person working the pattern would experience very high sleepiness at some point during the shift.
  • Risk Index (RI) — a modelled estimate of the relative risk of an incident compared with a reference pattern of work.

The two indices are similar in many respects but diverge, largely because fatigue and risk peak at different times of day. HSE’s calculator guidance recommends reviewing both. For a plain-English comparison, see Fatigue Index vs Risk Index.

Both are modelled estimates of relative exposure — not measurements of a specific person, and not legal FI/RI limits.

How RR446 relates to shift work

Section titled “How RR446 relates to shift work”

RR446 gives shift-based organisations a research-based, consistent way to compare work patterns. It reflects the reality that fatigue depends on how work is organised — timing, duration, recovery, and workload — not only total hours. This aligns with wider HSE shift work guidance in HSG256.

The model can help identify which patterns or sequences are likely to be more tiring, supporting roster design and fatigue risk assessment.

How it should — and should not — be interpreted

Section titled “How it should — and should not — be interpreted”

Reasonable use:

  • As decision-support to compare patterns and target improvements
  • As one input to a governed process, interpreted by competent persons
  • With an understanding of the model’s assumptions and limitations

Misinterpretations to avoid:

  • Treating FI or RI as a legal limit or a pass/fail test — RR446 outputs are not “satisfactory/unsatisfactory” verdicts
  • Assuming a value below any historical indicative threshold means nothing more need be done — staff may still be fatigued, and it is often reasonably practicable to improve further
  • Treating the model as a measurement of real-time alertness or fitness for duty
  • Presenting RR446 as current HSE-endorsed software (see the HSE FRI calculator withdrawal)

ORR guidance is explicit that these tools make a mathematical prediction — they do not “know” the fatigue staff will actually experience, and outputs should not be used in isolation.

Why RR446 is research, not a compliance threshold

Section titled “Why RR446 is research, not a compliance threshold”

RR446 provides a method. It does not set mandatory scores, and HSE does not currently specify threshold FI values that organisations must meet. Any colour bands or trigger levels are organisational policy thresholds — configured to context and validated against an organisation’s own activities and safety data so far as reasonably practicable.

Legal duties around fatigue arise from health and safety law and working time rules — not from an index value. This site does not provide legal advice on those duties.

Section titled “Related pages”

References

Section titled “References”