If the staffing level is lower than optimal then the employees in that operation have to work additional hours or extra shifts to keep the positions filled. These hours may be added on by:

  1. Holding employees over for additional hours at the end of their shift (i.e. effectively increasing the actual shift length and reducing off-duty rest hours between shifts).
  2. Calling employees in early for additional hours at the beginning of their shift (i.e. increasing the actual shift length and reducing off-duty rest hours before the shift).
  3. Scheduling employees to work on their days off to cover open shifts (thereby increasing the number of consecutive workdays and/or reducing the number of consecutive days off)
  4. Short notice call-in to cover vacant positions (creating the potential to miss or compromise a planned sleep period and thus start the shift unrested and/or less fit for duty).
  5. Having employees work double or even triple shifts (increasing actual shift length and reducing off-duty rest hours after the shift).

As a result, the amount of overtime worked by employees will be significantly increased, and the additional hours and days worked will make the published shift schedule a work of fiction. Up to a point, overtime is often welcomed, if not desired by many employees, as an understandable way to increase their paychecks. Some employees will volunteer for all of the overtime they can get, which buffers those who don’t want the extra hours. This also makes life easier for their supervisors. However, from every scientific and operational perspective, any significant understaffing, especially when there is uneven distribution of overtime, will affect both acute and chronic fatigue levels, and can represent a high-risk occupational health and safety exposure. 

This white paper does not advocate a goal of zero overtime. After all, when distributed and managed efficiently, overtime provides a measure of operational flexibility and it gives people a chance to earn additional pay. Instead the objective of the white paper is to educate managers at 24-hour operations on the causes and consequences of understaffing, and to help them address staffing

It’s an eternal debate among shift work managers, and a question we get asked all the time here at CIRCADIAN®.

Which are better: 8-hour shifts or 12-hour shifts? Which shift length produces the greatest productivity? The fewest safety incidents? The highest worker satisfaction? The fewest health problems?

The simple answer is: There’s no simple answer. Extensive scientific research on the effects of 8- and 12-hour shifts has produced no clear winner. Both 12s and 8s work wonderfully for some facilities while causing problems for others.

As further evidence that one schedule is not clearly better than the other, look at the distribution of shift schedules in North America. In a CIRCADIAN survey of 400 shift work operations, 37 percent were using 12-hour shifts while 34 percent were using 8-hour shifts (see graph below).

Shift-Schedules in North America

We do know this: The “perfect shift schedule ” for each facility depends on a large number of factors, including business needs, the nature of the job tasks performed, and workforce demographics and preferences.

The Research: 8s vs. 12s

A research review of “8s vs. 12s,” which examined over 60 research studies, rated 8s and 12s in terms of performance, safety, health, worker satisfaction, absenteeism and overtime (Smith L, et al, 1998). Here’s what the review found:

Performance. The evidence comparing the effects of 8s and 12s on job performance is unclear. Several studies of nurses concluded that there are generally no differences in fatigue or critical thinking performance in those working either shift schedule. However, three studies found that nurses’ performance tends to fare worse when they’re on 12s than on 8s. One of these three studies reported that the reduced performance appeared to be acute and related to the first 12-hour shift after a block of days off.

Two studies found that performance stayed the same within the mining, chemical, and petroleum industries.

Several studies mentioned that 12s are popular among shiftworkers because they compress the work week and provide more days-off than 8’s. Several studies attributed the popularity of 12s as improving staff morale and reducing absenteeism.

Additionally, the popularity of 12’s also been hypothesized to increase worker motivation and stimulate greater effort to reduce any possible detrimental effects of increased fatigue on 12-hour schedules.

Safety. The vast majority of studies that compared accident and error rates before and after a schedule change found no difference between 8s and 12s. Studies on this topic have included nuclear power plants, petrochemical companies, fertilizer producers, utilities, and processing plants.

In one study, a decrease from three to two shift handovers per day following a switch from 8s to 12s was thought to have reduced operator error. On the other hand, two studies cited accumulated sleep debt and recommend against 12-hour shifts, especially schedules entailing more than three to four 12-hour shifts in a row.

Health. Most studies have found few differences between 8s and 12s when it comes to physical health, social well-being, and sleep. Studies that examined worker satisfaction with family life favored 8-hour shifts for nurses and 12-hour shifts for police and chemical workers.

A shift schedule’s effect on health and social issues will vary considerably based on individual differences, which is why it’s important to consider the characteristics of each worker population during schedule selection.

Worker satisfaction. Studies overwhelmingly agree that the way an operation implements a shift schedule has a substantial impact on the employees’ acceptance of it. Employee participation and sense of control is crucial to their satisfaction with and attitude toward a new schedule (click here to read white paper on "Shift Scheduling & Employee Involvement").

Facilities in various industries reported positive results when employees participated in negotiating changes such as shift start and end times, provision of an extra break during 12-hour shifts, and improved meal facilities at night. At one plant in which the schedule was imposed with no employee input, worker reactions were so negative that the old schedule had to be reinstated.

Absenteeism. Six studies found no difference in absenteeism rates between 8s and 12s. One plant saw an increased applicant rate and decreased turnover after switching to 12s. However, two studies found that older workers had less favorable attitudes toward 12-hour shifts, leading to higher absence rates among older workers who participated in that study.

Overtime. The amount of overtime does not generally differ between 8s and 12s. However, companies using 12-hour shifts may want to adjust overtime policies so that workers are not held over for dangerously long periods. Several studies suggest that safety incidents go up when the average annual overtime worked by employees goes up, an effect thought to be produced by fatigue from working long hours.

In addition, a 12-hour shift schedule often allows employees greater opportunities for second jobs; one study found 25 percent of shiftworkers on 12-hour schedules were moonlighting, which may take energy away from their primary jobs.


In summary, to quote the authors of the research review, “The research findings are largely equivocal. The bulk of the evidence suggests few differences between eight and 12 hour shifts in the way they affect people.”

Neither 8s nor 12s pull out ahead in terms of performance, safety, health, worker satisfaction, absenteeism, or overtime. One clear conclusion is that employee buy-in and satisfaction with a new work schedule improves when they are given input into the selection and implementation of the new schedule.

When reading these study results it’s important to remember that one study’s conclusion about (for example) a “2-3-2” shift pattern may not apply to all 12-hour shift patterns. For both 8s and 12s, there are hundreds of different shift patterns for 24/7, 24/6 and 24/5 facilities, and each study typically only examines one pattern.

That’s one reason why it’s difficult to draw conclusions about whether 8s or 12s are best. You can’t compare apples to apples if two studies of 12-hour shifts each look at a different shift pattern — or a different industry — or a different number of employees — or any of dozens of variable factors.

So which schedule is right for your operation? A thorough analysis of your company’s own needs and your workers’ preferences will let you determine the optimum schedule for your facility. To learn more, please visit our Shift Scheduling Optimization Process page or call us at 1-800-284-5001 or 781-439-6300.

Smith L, Folkard S, et al. Work shift duration: a review comparing eight hour and 12 hour shift systems. Occup Environ Med 1998;55:217-229.
Aguirre A, Moore-Ede A. Shiftwork Practices 2007. Circadian Information LP, 2008.

Next Steps:

 Shift Scheduling White Papers:

 Have a question? Contact a CIRCADIAN shift scheduling expert:

Sound can invigorate you or send you to sleep.

Don’t believe us? Then just close your eyes for a few seconds and imagine the sounds of a rolling surf on the beach, or the steady pitter-patter of a rain storm and you’ll probably be lulled to sleep in no time. In fact, these sounds are so effective that they are now electronically simulated in “white noise” machines that many people use in their bedrooms. 

Unfortunately, the same electronic “white noise” is produced in less desirable places, such as industrial control rooms, production areas, and even in our cars and trucks, by the equipment that people are meant to watch alertly through the night. 

With that in mind, experienced shiftworkers recommend that you…

Advice #1 - You will need to approach sleep differently.  Before shiftwork, chances are sleep was most likely something you just did at night. However, as a shiftworker you will be introduced to the challenges of trying to sleep during the day and possibly also to the challenge of having a rotating bedtime.

To Counteract Fatigue, it is Important be Familiar with Some of its Main Causes and Symptoms.

Common Causes of Fatigue

  1. Sleep deprivation: Not getting enough sleep is probably the most common cause of fatigue. It’s important to keep in mind that there are different reasons for being sleep deprived, including:
    • Extended time awake. How long has it been since you slept? After being awake for 16 consecutive hours most people begin to feel tired.
    • Reduced time asleep. Most people need between 7-9 hours of sleep to be at their best.  
    • Disrupted or poor quality sleep. Whether it’s a baby waking you up every 2 hours or back pain that prevents you from getting comfortable, not getting good quality sleep can lead to sleep deprivation.
    • The cumulative effect of multiple days with shortened or disrupted sleep – Has it been a couple of days since you last slept well? This type of sleep deprivation could put you at greater risk for feeling fatigued. 

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