An employer should take all reasonable steps to ensure that the normal working hours of a nightworker do not exceed an average of eight hours in any 24-hour period. This is averaged over a 17-week reference period. A nightworker is a person who normally works at least three hours during night time (defined as between 11pm and 6am).
If the nightwork involves 'special hazards, or heavy physical or mental strain', a strict eight-hour time limit is imposed on working time in each 24-hour period and no averaging is allowed over a reference period.
Note that there are no 'opt-outs' on working time regulations for nightworkers.