(1) In this section -

"night time'' means the period between midnight and 7 a.m. on the following day;

"night work'' means work carried out during night time;

"night worker'' means an employee -

(a) who normally works at least 3 hours of his or her daily working time during night time,

and

(b) the number of hours worked by whom during night time, in each year, equals or exceeds 50 per cent. of the total number of hours worked by him or her during that year.

(2) Without prejudice to section 15, an employer shall not permit a night worker, in each period of 24 hours, to work -

(a) in a case where the work done by the worker in that period includes night work and the worker is a special category night worker, more than 8 hours,

(b) in any other case, more than an average of 8 hours, that is to say an average of 8 hours calculated over a period (hereafter in this section referred to as a ''reference period'') that does not exceed -