The night work limit is the 48 hour week wearing a different hat
"Night workers can only do eight hours" is the most misquoted rule in shift work, and the arithmetic is worth doing once because it changes what you build.
Regulation 6(1) caps a night worker's normal hours at an average of eight for each 24 hours, over the same 17 week reference period as everything else.Regulation 6(3) works that average out as the normal working hours divided by the 24 hour periods in the reference period, less the days of weekly rest.
Put the statutory minimums in and it collapses:
- Over 17 weeks there are 119 days
- Weekly rest takes out 17 of them
- So the average is 17W ÷ 102, which is W ÷ 6
And W ÷ 6 goes over eight exactly when W goes over 48.
The night limit and the 48 hour week are the same constraint. It is not a tighter separate rule sitting on top, and a twelve hour night shift breaches nothing at all by being long. The tool shows both figures side by side so you can watch them cross the line together.
The clearest demonstration is 4 on, 3 off at twelve hours. Four shifts in seven days is exactly 48 hours a week, and exactly eight night hours in each 24. Two supposedly unrelated rules land on the same boundary at the same instant, because they are the same sum.
Regulation 6(7) is the one that bites
Where night work involves special hazards, or heavy physical or mental strain, the eight hours stops being an average and becomes an absolute limit in any 24 hour period. No reference period. No averaging. No opt-out.
That rules out every twelve hour night pattern on that kind of work, outright. It is the single switch most likely to make a rota unlawful, and almost nobody quotes it.
Whether work falls inside it is decided by a collective or workforce agreement covering the effects of night work, or by the employer's own risk assessment under the Management of Health and Safety at Work Regulations 1999. So the answer usually already exists in a document somewhere, written by somebody who did not realise they were deciding the shift pattern.
Daily rest fails at the changeover, not inside the block
Eleven hours between shifts sounds like the rule most likely to catch a twelve hour pattern out. It is not. Four twelve hour days in a row leaves a twelve hour gap every night and passes comfortably.
It fails where the pattern turns over. A late finishing at 22:00 followed by an early starting at 06:00 is eight hours, and both of those are ordinary shifts that look entirely correct written on their own line. The gap between them appears nowhere on the rota.
So the tool walks the whole cycle as one timeline, and it wraps round the end back into the start. That join is real: it happens every cycle, for as long as the rota runs, and it is the one most often missed because the printed rota stops at the bottom of the page.
Why so many patterns land on 42 hours
4 on 4 off, 2-2-3, DuPont and continental all come out at 42 hours a week by completely different routes. That is not a coincidence.
Forty two gives continuous cover with four crews while staying far enough under the 48 hour average that nobody has to sign an opt-out. Seven days at 24 hours is 168 hours to cover; four crews at 42 is exactly that. The patterns differ in how the days off are arranged, not in how many there are.
That matters when comparing them: the real difference between 4 on 4 off and DuPont is not hours or pay. It is whether you would rather have four days off every eight, or seven in a row once a month.
A workforce agreement can move the night period
This reports the default position. It does not account for a workforce agreement setting a different night period or reference period, the sector exemptions, or workers whose time is not measured or predetermined. Road transport has its own regime entirely.
Under 18s cannot work most of these patterns at all: they have hard limits of 8 hours a day and 40 a week with no averaging and no opt-out, and 12 hours of daily rest rather than 11. The rest breaks checker handles that properly, and the 48 hour working time check covers what happens to the average across a real 17 weeks including leave.
Common questions
Can night workers really only work 8 hours?
Not the way it is usually quoted. Regulation 6(1) sets an AVERAGE of 8 hours in each 24 over a 17 week reference period, not a limit on the length of a shift. Work regulation 6(3) with the statutory minimums and it comes out at 48 hours a week, which is the same place the ordinary weekly limit already sits. So a 12 hour night shift breaches nothing by being long.
So when does the 8 hour night limit actually bite?
Regulation 6(7). Where night work involves special hazards, or heavy physical or mental strain, the 8 hours stops being an average and becomes an absolute limit in any 24 hour period. No reference period, no averaging, no opt-out. That rules out every 12 hour night pattern on that kind of work.
How do I know if my work counts as special hazards or heavy strain?
It is decided by a collective or workforce agreement covering the effects of night work, or by the employer’s own risk assessment under the Management of Health and Safety at Work Regulations 1999. So somebody has usually already answered it, often without noticing that the answer decided the rota.
Why do so many shift patterns come out at 42 hours a week?
Because 42 gives continuous cover with four crews while staying comfortably under the 48 hour average, so nobody has to sign an opt-out. 4 on 4 off, 2-2-3, DuPont and continental all land there by different routes.
Which patterns break the 11 hour rest rule?
Almost never the block, almost always the changeover. Four twelve hour days in a row gives a twelve hour gap every night and is fine. A late finishing at 22:00 followed by an early starting at 06:00 is eight hours and is a breach, and both of those are ordinary shifts that look correct written on their own line.
What makes somebody a night worker?
Working at least three hours of their daily working time during the night period as a normal course. The night period is at least seven hours including midnight to 5am, and it defaults to 11pm to 6am where no agreement says otherwise.
Does the 48 hour opt-out cover the night limits too?
The average one, effectively, since they are the same constraint. The absolute limit in regulation 6(7) cannot be opted out of at all, and neither can daily or weekly rest. The opt-out only ever touched the 48 hour average.