Definition of shiftwork

Definition of shiftwork 

The International Labour Office (International Labour Organization, 1990a) defines 

working in shifts as “a method of organization of working time in which workers succeed 

one another at the workplace so that the establishment can operate longer than the hours 

of work of individual workers.” 

The European Council Directive 93/104 (1993) declares that “concerning certain 

aspects of the organisation of working time, shiftwork shall mean any method of 

organising work in shifts whereby workers succeed each other at the same work stations 

according to a certain pattern. Shiftworker shall mean any worker whose work schedule is 

part of shiftwork.” 

Besides these definitions, in the scientific literature, the term “shiftwork” has been 

widely used and generally includes any arrangement of daily working hours other than the 

standard daylight hours (7/8 am – 5/6 pm). 

In most cases, shiftwork is synonymous of irregular, odd, flexible, variable, unusual, 

non-standard working hours. 

Types of shiftwork 

Several types of shiftwork exist and can be described as follows: 

(a) permanent – people work regularly on one shift only, i.e. morning or 

afternoon or night; or rotating – people alternate more or less periodically on different 

shifts; 

(b) continuous – all days of the week are covered; or discontinuous – interruption 

on weekends or on sundays; 

(c) with or without night work – the working time can be extended to all or part of 

the night, and the number of nights worked per week/month/year can vary 

The shift systems can also differ widely in relation to other organizational factors: 

(a) length of shift cycle – a “cycle” includes all shifts and rest days lasting as long 

as the series of shifts restart from the same point; there can be short (6–9 days), 

intermediate (20–30 days), or long (up to 6 months or more) cycles. 566 IARC MONOGRAPHS VOLUME 98  

(b) duration of shifts – in general, the length of a shift is 8 hours, but can range 

from 6 to 12 hours. 

(c) number of workers/crews who alternate during the working day. 

(d) start and finish time of the duty periods. 

(e) speed of shift rotation – this depends on the number of consecutive days 

worked before changing shift. It can be fast (i.e. every 1, 2 or 3 days), intermediate 

(i.e. every week), or slow (i.e. every 15, 20 or 30 days). This factor has considerable 

influence on the number of consecutive night shifts and rest days. 

(f) direction of shift rotation – it can be clockwise (i.e. morning/afternoon/night) 

or counter-clockwise (i.e. afternoon/morning/night) with consequent different 

duration of the intervals between shifts. Clockwise rotation is also referred to as 

“phase delay” or “forward rotation,” and counter-clockwise rotation, “phase advance” 

or “backward rotation”. They have a different impact on the adjustment of the 

circadian rhythm. 

(g) number and position of rest days between shifts. 

(h) regularity/irregularity of the shift schedules. 

All of these factors can be combined in different ways depending on the demands 

specific to the occupation. 

In the industrial sectors (i.e. mechanical and chemical), shiftwork is usually arranged 

in continuous three-shift systems. A similar number of crews/workers work both on day 

and night shifts, with regular shift schedules either on fast or slow rotating cycles, with 

fixed start and finishing times. 

In the transport sector, schedules are often quite irregular, both in terms of number of 

consecutive shifts, shift rotation, start and finishing times, duration of the duty periods, 

location, and amount of rest days. 

In the health-care sector, quite different shift schedules are operated with different 

rotation (clockwise or counter-clockwise), variable start and finishing time, and different 

amount of night shifts. 

In the service sector, workers are commonly employed on split shifts, for example, 

very early morning and late afternoon shifts in road- and office-cleaning, merchandise 

delivery, or permanent night work (security guards). 

In the leisure sector, work is mainly performed during the late afternoon and night 

hours, with a long duration of shifts. 

Different shiftwork systems have potentially different impacts on the health of the 

workforce, disturbing the circadian rhythm, an essential biological function, in different 

ways, and also inducing sleep deprivation  (see Section 4). In addition to shiftwork 

schedules, other factors can affect tolerance to shiftwork and night work such as 

individual characteristics, family situation, social conditions, and working conditions 

(Fig. 1.1; Costa  et al., 1989; Costa, 1996, 2003; Knauth, 1996; Knauth & Hornberger, 

2003). 

Scientific guidelines 

The main indications for the design of better shift systems according to ergonomic 

criteria are (Knauth, 1996; Knauth and Hornberger, 2003; Wedderburn, 1994): 

a) Quickly rotating shift systems are better than slowly rotating ones. 

b) Clockwise rotation (morning/afternoon/night) is preferable to counterclockwise (afternoon/morning/night). 

c) Early starts for the morning shift should be avoided. 

d) Prolonged work shifts (9–12 hour)  should only be considered when the 

workload is suitable, there are adequate breaks, and the shift system is designed to 

minimize accumulation of fatigue and exposure to toxic substances. 

e) Shift systems should be regular and able to guarantee as many free weekends 

as possible. 

f) Permanent night work can be acceptable only for particular working situations 

which require a complete adjustment to night work to guarantee the highest levels of 

safety. Be aware that such complete adjustment requires people to maintain the 

inverted sleep/wake cycle also on rest days and to avoid exposure to bright light after 

night shifts (i.e. wearing dark sun glasses while commuting home). 

g) Adequate time off between shifts should be allowed to compensate for fatigue 

and sleep as quickly as possible (i.e. two shifts in the same day must be avoided), and 

rest days should come preferably after the night duty period to allow prompt recovery 

from sleep deficit and an easier return to the normal sleep/wake cycle. 

h) Some flexibility in working times is desirable to give the workers the 

possibility of combining better work duties with family and social life.