Calculating your night work hours

If you are a night worker you should not work more than an average of eight hours over a night-time period. This is generally regarded as 11 00 pm – 6 00 am.

Working time reference period

Your average night working time is normally calculated over a 17 week ‘reference period’ or however long a worker has been working for their employer if this is less then 17 weeks. This can be changed with agreement from your employer as part of a 'collective agreement', which is an agreement reached between your employer and a trade union.

Trade unions: an introduction

Calculating simple working patterns

Your average night-time working hours are calculated by dividing the number of your normal hours worked in a reference period, by the number of days in the period. This does not include the number of rest days you are entitled to take.

Example one

If you are a night worker and work four 12-hour shifts each week for example:

multiply the number of weeks in the reference period (17) with the number of hours you work each week: 17 x (4 x 12) = 816
in a 17 week period there are 119 days (17 x 7). You are entitled to take 17 weekly rest periods, therefore the number of days you could be asked to work is: 119 – 17 = 102 days
Your daily average working time is your total hours divided by the number of days you could be required to work: 816 ÷ 102 = 8

So you would have worked an average of eight hours a day, which would be within the night work limit.

Calculating shift patterns

Sometimes shift working patterns are not as simple to work out as you may have taken leave.

Example two

You are a night worker and work a ‘shift cycle’ of five days of 10 hour shifts which is followed by three days of rest. The cycle starts at the beginning of the reference period. You also take two weeks leave and work six hours of overtime every five weeks (in the fifth, tenth and fifteenth weeks).

calculate how many shift cycles the reference period covers by dividing the number of days in your reference period (17 weeks) with the number of days in your shift cycle: 17 x 7 = 119 then 119 ÷ 8 = 15 cycles (rounded to the nearest number)
next total the number of hours worked in the cycles: 15 x (5 days x 10 hour shifts) = 750 hours
add the overtime which is 6 hours overtime x 3 weeks = 18 + 750 = 768 hours worked
subtract the weekly rest period you must take (rather than the amount your employer allows you) to calculate the number of days you could be asked to work: 119 – 17 = 102 days
divide the total hours worked by the number of days you could have worked: 768 ÷ 102 = 7.53

So you would have worked an average of 7.53 hours a day, which is within the night work limit.

The annual leave taken does not affect the calculation of average working hours.

Do you have a problem with your night worker rights?

If you feel you are exceeding the night work limits:

your employer may be in breach of contract
the Health and Safety Executive for Northern Ireland could take action against your employer

If you think you are not receiving any other rights as a night worker you can:

talk to your manager to try to sort matters quickly - be aware that your employer must keep records of the hours you have worked for them
contact your trade union or health and safety representative (if you have one) for advice
either approach matters informally or follow your grievance procedure.