The HSE Fatigue Risk Index: what happened and what comes next

The Health and Safety Executive's Fatigue Risk Index (FRI) has been the de facto standard for assessing shift-work fatigue in UK safety-critical industries since the early 2000s. Developed from biomathematical models of sleep and circadian rhythm, it gave operators a single number — a relative measure of risk associated with a given schedule — and for two decades that number was good enough. It informed rail timetabling, road haulage planning, nuclear control-room rotas, and aviation crew rosters. It made fatigue visible in a domain that had previously treated it as an unmanaged residual.

But the FRI was built for a steadier world. It assumes regular schedules, predictable rest, and workers whose lives outside the gate look something like the model expects. It does not account for the modern reality of split shifts, on-call patterns layered onto nominal rotas, multi-employer working in the gig economy, or the fragmented sleep most shift workers actually achieve. Its outputs are point estimates, not distributions, and they do not update as circumstances change. A roster that scores well on Friday can be unsafe by Tuesday if a colleague calls in sick and the swap cascades.

In this piece we'll trace where the FRI came from, where the science has moved in the twenty years since, and what a successor framework — one that uses the data operators already collect — actually needs to look like.