Shift work

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Organized labour

Shift work is an employment practice designed to make use of or provide service for respectively the 24 hours of the clock per each day of the week (abbreviated as 24/7).

The term "shift work" includes both long-term night shifts and work schedules in which employees change or rotate shifts.[1][2]

A related yet different concept, the work shift, is the time period during which a person is at work.

Contents

Health consequences

Main article: Shift work sleep disorder

The February 15, 2005 issue of American Family Physician noted that shift work has been associated with cluster headaches. Health problems in the short term can also include fatigue, stress and loss of concentration, a higher rate of absence from the job and poor sexual performance, as shown in the majority of 200 variable-shift workers in a recent study in Kuwait.[3]

Long term consequences of disturbing natural circadian rhythms have been investigated also. A study by Knutsson et al. in 1986 found that shift workers who had worked in that method for 15 years or more were 300% more likely to develop ischemic heart disease.

In 1978 Cohen et al. proposed that reduced production of the hormone melatonin might increase the risk of breast cancer, citing "environmental lighting" as a possible causal factor.[4] Working the night shift first became associated with higher rates of cancer in 1987. This may be due to alterations in circadian rhythm: melatonin, a known tumor suppressant, is generally produced at night and late shifts may disrupt its production. Multiple studies have documented a link between night shift work and the increased incidence of breast cancer.[5][6][7][8]

The WHO's International Agency for Research on Cancer listed "shiftwork that involves circadian disruption" as a probable carcinogen in 2007 (IARC Press release No. 180).[9][10]

UCSF neurologist Louis Ptacek, who studies circadian rhythms, genes and sleep behaviors, has been quoted: "It's not surprising, we have evolved on a planet that is rotating every 24 hours. Our internal clock is more than just when we sleep and wake. It's related to cell division and it regulates our immune systems. When we battle our internal clock, that has complications."[11]

A good review of current knowledge of the health consequences of exposure to artificial light at night and an explanation of the causal mechanisms was published in the Journal of Pineal Research in 2007.[12]

One study suggests that, for those working a night shift (such as 23:00 to 07:00), it may be advantageous to sleep in the evening (14:00 to 22:00) rather than the morning (08:00 to 16:00). The study's evening sleep subjects had 37% fewer episodes of attentional impairment than the morning sleepers.[13]

The health consequences of shift work may depend on one's chronotype, that is, whether one is a day person or a night person, and what shift one is assigned to.

Shift work management practices

The practices and policies put in place by managers of round-the-clock or 24/7 operations can significantly influence shift worker alertness (and hence safety) and performance.

Air traffic controllers typically work an 8-hour day, 5 days per week. Research has shown that when controllers remain "in position" for more than two hours, even at low traffic levels, performance can deteriorate rapidly, so they are typically placed "in position" for 30-minute intervals (with 30 minutes between intervals).

These practices and policies can be fairly obvious: selecting an appropriate shift schedule or rota and using an employee scheduling software to maintain it, setting the length of shifts, managing overtime, increasing lighting levels, or providing shift worker lifestyle training to help shift workers better handle issues such as understanding basic circadian physiology, sleep and napping, caffeine usage, social life issues, diet and nutrition, etc. They may also be more indirect: retirement compensation based on salary in the last few years of employment (which can encourage excessive overtime among older workers who may be less able to obtain adequate sleep), or screening and hiring of new shift workers that assesses adaptability to a shift work schedule.

A day may be divided into three shifts, each of 8 hours, and each employee works just one of those shifts; they might, for example, be midnight to 08:00, 08:00 to 16:00, 16:00 to midnight. Generally, "first shift" refers to the day shift, with "second shift" running from late afternoon to midnight or so, and "third shift" being the night shift. On occasion, more complex schedules are used, sometimes involving employees changing shifts, in order to operate during weekends as well, in which case there will be four or more sets of employees.

12-hour work shifts are also in use. In a modern steelworks, four sets of personnel are used, working consecutive days in one 12-hour shift (06:00 – 18:00 and vice-versa). Shift A will work days, and shift B nights, over a 48-hour period, before handing over to shifts C and D and taking 48 hours off. In the offshore petroleum industry, employees may work 14 consecutive days or nights, 06:00 – 18:00 or 18:00 – 06:00, followed by three or four weeks free. The svingskift (literally: "swing shift") in the offshore petroleum industry in Norway refers to a two-week tour during which employees work 12-hour days the first seven days and 12-hour nights the second (or vice versa).

Shift work was once characteristic primarily of the manufacturing industry, where it has a clear effect of increasing the use that can be made of capital equipment and allows for up to three times the production compared to just a day shift. It contrasts with the use of overtime to increase production at the margin. Both approaches incur higher wage costs. Although 2nd-shift worker efficiency levels are typically 3–5% below 1st shift, and 3rd shift 4–6% below 2nd shift, the productivity level, i.e. cost per employee, is often 25% to 40% lower on 2nd and 3rd shifts due to fixed costs which are "paid" by the first shift.[14]

In general, requiring workers to live on a time-shifted schedule for extended periods is unpopular, and this typically must be paid for at a premium. It is common in heavy industry, particularly automobile and textile manufacturing and is becoming more common in locations where a shut-down of equipment would incur an extensive restart process. Food manufacturing plants, in particular, have extensive cleaning programs that are required before any restart. The use of shift work in manufacturing varies greatly from country to country. Shift work is common in the transportation sector as well. Some of the earliest instances appeared with the railroads, where freight trains have clear (i.e. passenger-free) tracks to run on at night. Shift work has been traditional in law enforcement and the armed forces: for example sailors must be available to handle a vessel around the clock, and a system of naval watches organized to ensure enough hands are on duty at any time. This is shift work by another name.

Service industries now increasingly operate on some shift system; for example a restaurant or convenience store will normally each day be open for much longer than a working day. Shift work is also the norm in governmental and private employment in fields related to public safety and healthcare, such as Emergency Medical Services, police, fire prevention, security and hospitals. Companies working in the field of meteorology, such as the National Weather Service and private forecasting companies, also utilize shift work, as constant monitoring of the weather is necessary.

Much of the Internet services and Telecommunication industry relies on shift work to maintain worldwide operations and uptime.

Shift patterns

6 on 6 off

Consists of 3 days and 3 nights.Then 6 days off. These will alternate between other crews A.K.A teams for a full 24/7 operation.

Three-shift system

The "three-shift system" is the most common pattern, with "first" from 06:00 to 14:00, "second" from 14:00 to 22:00, and a "third" (or "night") shift from 22:00 to 06:00 This is generally worked over a five-day week; to provide coverage 24/7, employees have their days off ("weekends") on different days.

All of the shifts have desirable and less desirable qualities. First shift has very early starts, so time in the evening before is heavily cut short. The second shift (or "swing shift") occupies the times during which many people finish work and socialize. The third shift creates a situation in which the employee must sleep during the day.

Generally, employees stay with the same shift for a period of time, as opposed to cycling through them; this is seen as healthier.

Three-shift example:

Time Sat Sun Mon Tues Wed Thur Fri
06:00–14:00 Shift 1 Shift 1 Shift 1 Shift 1 Shift 1 Off Off
14:00–22:00 Shift 2 Shift 2 Shift 2 Shift 2 Shift 2 Off Off
22:00–06:00 Shift 3 Shift 3 Shift 3 Shift 3 Shift 3 Off Off

Four on, four off

Four on, four off is a shift pattern that is being heavily adopted in the United Kingdom and in some parts of the United States. An employee works for four days, usually in 12-hour shifts (7:00 to 7:00) then has four days off. While this creates a "48-hour week"(42-hour average over the year) with long shifts, it may be preferred because it shrinks the work week down to four days, and then gives the employee four days rest—double the time of a usual weekend. Due to the pattern, employees effectively work an eight-day week, and the days they work vary by "week". As with three-shift system, most employees stay with the same shift rather than cycling through them.

Four on, four off example:

Time 7:00-7:00 Sat Sun Mon Tues Wed Thur Fri Sat
07:00–19:00 Day 1 Day 2 Day 3 Day 4 Off Off Off Off

Navy Three-shift underway system

The US Navy uses a three shift system that relies on an 18 hour day instead of a 24 hour day. The 24 hour period is broken up into 4 shifts. 00-06, 06-12, 12-18, and 18-00. A sailor will stand watch on his shift. During his offgoing shift he will have time to perform maintenance, work on qualifications and handle all his collateral duties. During the oncoming time the sailor has time to sleep, relax, and do whatever personal things he needs to do like laundry.

Navy Three-shift example: Shift 1

Time Sat Sun Mon Tues Wed Thur Fri
00:00–06:00 Shift 1 Offgoing Oncoming Shift 1 Offgoing Oncoming Shift 1
06:00–12:00 Offgoing Oncoming Shift 1 Offgoing Oncoming Shift 1 Offgoing
12:00–18:00 Oncoming Shift 1 Offgoing Oncoming Shift 1 Offgoing Oncoming
18:00–00:00 Shift 1 Offgoing Oncoming Shift 1 Offgoing Oncoming Shift 1

Navy Three-shift example: Shift 2

Time Sat Sun Mon Tues Wed Thur Fri
00:00–06:00 Oncoming Shift 2 Offgoing Oncoming Shift 2 Offgoing Oncoming
06:00–12:00 Shift 2 Offgoing Oncoming Shift 2 Offgoing Oncoming Shift 2
12:00–18:00 Offgoing Oncoming Shift 2 Offgoing Oncoming Shift 2 Offgoing
18:00–00:00 Oncoming Shift 2 Offgoing Oncoming Shift 2 Offgoing Oncoming

Navy Three-shift example: Shift 3

Time Sat Sun Mon Tues Wed Thur Fri
00:00–06:00 Offgoing Oncoming Shift 3 Offgoing Oncoming Shift 3 Offgoing
06:00–12:00 Oncoming Shift 3 Offgoing Oncoming Shift 3 Offgoing Oncoming
12:00–18:00 Shift 3 Offgoing Oncoming Shift 3 Offgoing Oncoming 'Shift 3
18:00–00:00 Offgoing oncoming Shift 3 Offgoing Oncoming Shift 3 Offgoing

Four on, three off

Main article: Four-day week

Each employee works four days and gets a three-day weekend. For some types of manufacturing, this is a win-win arrangement. For example, Kyanize Paint Company had been making 3 batches of paint per day, Monday through Friday (3 × 5 = 15). They changed to making 4 batches of paint, Monday through Thursday (4 × 4 = 16). Total worker hours remained the same, but profits increased. In exchange for two additional hours of work per day, over 4 days, workers got an additional day off every week (see also the book, 4 Days, 40 Hours).

Four on, two off

In four on, two off the employee only gets two days off. In a seven-day period, this adds up to 60 hours worked (on average, based on 12-hour shifts). Four on, two off is mainly adopted by industries in which employees do not engage in much physical activity.

Four on, two off example:

Time Sat Sun Mon Tues Wed Thur Fri
07:00 to 19:00 Day 1 Day 2 Day 3 Day 4 Off Off Day 5

Four on, one off

In four on, one off the employee only gets one day off. In a seven-day period, this adds up to 48 hours worked (on average, based on 8-hour shifts). Four on, one off is mainly adopted by industries in which companies prefers to work for all days of the week with four shifts and where laws do not let employees work for 12 hours a day for several days.

Four on, one off example:

Time Sat Sun Mon Tues Wed Thur Fri
07:00–15:00 Day 1 Day 2 Day 3 Day 4 Off Day 5 Day 6

Two days, two nights, four off

A variation is the two days, two nights, four off pattern of working. In this shift schedule, employees work 12-hour shifts from 06:00 to 18:00 on day shifts and from 18:00 to 06:00 on nights. This pattern is currently in use by HM Coastguard in the UK, and employs four separate teams to maintain 24/7 coverage.

Two days, two nights, four off example:

Time Sat Sun Mon Tues Wed Thur Fri Sat
06:00–18:00 Day 1 Day 2 Off Off Off Off Off Off
18:00–06:00 Off Off Day 3 Day 4 Off Off Off Off

5/4/9s

"5/4/9s" (or "Five/Four Nines", or "9/80") is another variation. Employees work in 2 week cycles. Week 1 the employee works 4 days of 9 hours followed by 1 day of 8 hours with 2 days off (week 1: 44 hours). Week 2 the employee works 4 days of 9 hours with 3 days off (week 2: 36 hours). This pattern works to 80-hours in a 2 week pay-period (comparable to working 8 hours a day for 5 days a week). The benefit to working an extra hour a day gives you a "normal" 2 day weekend followed by a long 3 day weekend the next. Typical working hours for this type of shift would be 06:00 to 15:30 (9 hours with 30 minutes lunch) and 06:00 to 14:30 (8 hours with 30 minutes lunch) on the 8-hour work day. Often the employer will alter the starting times (e.g., start at 07:00 or 08:00). Also referred to as "9/80" as employees have nine days to work 80 hours.

5/4/9s shift example:

Week Mon Tues Wed Thur Fri Sat Sun
Week 1 9 hrs 9 hrs 9 hrs 9 hrs 8 hrs Off Off
Week 2 9 hrs 9 hrs 9 hrs 9 hrs Off Off Off

12/24/12/48

12/24/12/48 (or 12/24) is another variation. Employees work in shifts of 12 hours; first a daily shift (e.g., 07:00 to 19:00), followed by 24 hours rest, then a nightly shift (19:00 to 07:00), finishing with 48 hours rest. This pattern needs four teams for full coverage, and makes an average 42-hour workweek.

12/24/12/48 shift example:

Time Sat Sun Mon Tues Wed Thur Fri
06:00–18:00 Day 1 Off Off Off Day 3 Off Off
18:00 to 06:00 Off Day 2 Off Off Off Day 4 Off

Continental shift

Continental shift, adopted primarily in central Europe, is a rapidly changing three-shift system that is usually worked for seven days straight, after which employees are given time off. For example, three mornings, two afternoons, and then two nights.

Continental shift example:

Time Sat Sun Mon Tues Wed Thur Fri
06:00–14:00 Shift 1 Shift 1 Shift 1 Off Off Off Off
14:00 to 22:00 Off Off Off Shift 1 Shift 1 Off Off
22:00–06:00 Off Off Off Off Off Shift 1 Shift 1

Split shift

Main article: Split shift

Split shift is used primarily in the catering, transport, hotel, and hospitality industry. Waiters and chefs work for four hours in the morning (to serve lunch), then four hours in the evening (to serve an evening meal). The average working day of a chef on split shifts could be 10:00 to 14:00 and then 17:00 to 21:00

Split shift example:

Time Sat Sun Mon Tues Wed Thur Fri
10:00–14:00 On On On On On Off Off
14:00–17:00 Off Off Off Off Off Off Off
17:00–21:00 On On On On On Off Off

Earlies and lates

Earlies and lates is used primarily in industries such as customer service (help desk/phone-support), convenience stores, child care (day nurseries), and other businesses that require coverage greater than the average 9:00 to 5:00 working day in the UK. Employees work in two shifts that largely overlap, such as early shift from 08:00 to 16:00 and late shift from 10:00 to 18:00

Earlies and lates shift example:

Time Sat Sun Mon Tues Wed Thur Fri
08:00–16:00 Shift 1 Shift 1 Shift 1 Shift 1 Shift 1 Off Off
15:00–23:00 Shift 2 Shift 2 Shift 2 Shift 2 Shift 2 Off Off

7-day fortnight shift

In the 7-day fortnight shift pattern, employees work their allotted hours within 7 days rather than 10. Therefore, 41 hours per week equate to 82 hours per fortnight (fourteen days and nights), which is worked in seven days, at 11–12 hours per shift. This shift structure is used in the broadcast television industry, as well as many law enforcement agencies, as well as health care fields such as nursing and clinical laboratories in the US.

7-day fortnight shift example:

Week 1
Time Sat Sun Mon Tues Wed Thur Fri
Days
08:00 to 20:00		 Shift 'A'
Day 1		 Shift 'A'
Day 2		 Shift 'C'
Day 1		 Shift 'C'
Day 2		 Shift 'C'
Day 3		 Shift 'A'
Day 3		 Shift 'A'
Day 4
Nights
20:00 to 08:00		 Shift 'B'
Night 1		 Shift 'B'
Night 2		 Shift 'D'
Night 1		 Shift 'D'
Night 2		 Shift 'D'
Night 3		 Shift 'B'
Night 3		 Shift 'B'
Night 4
Week 2
Time Sat Sun Mon Tues Wed Thur Fri
Days
08:00 to 20:00		 Shift 'C'
Day 4		 Shift 'C'
Day 5		 Shift 'A'
Day 5		 Shift 'A'
Day 6		 Shift 'A'
Day 7		 Shift 'C'
Day 6		 Shift 'C'
Day 7
Nights
20:00 to 08:00		 Shift 'D'
Night 4		 Shift 'D'
Night 5		 Shift 'B'
Night 5		 Shift 'B'
Night 6		 Shift 'B'
Night 7		 Shift 'D'
Night 6		 Shift 'D'
Night 7

One of the advantages of using this pattern is each shift pair, for example A and B, will get time off on weekends alternatively, because the schedule is fixed and does not drift.

DuPont 12-Hour rotating shift

The DuPont 12-Hour rotating shift provides 24/7 coverage using 4 crews and 12-hour shifts while providing a week off. Average hours is 42 per week but contains a 72-hour week which can be challenging. It is used in several manufacturing industries in the US. Companies that have gone to this schedule have noticed a decrease in accidents plus more rest for employees, less call ins, and more coverage when crews are short handed. In all the schedule is designed to improve safety.

DuPont 12-Hour rotating shift example:

Week Fri Sat Sun Mon Tues Weds Thurs
1 Nights Nights Nights Nights Off Off Off
2 Days Days Days Off Nights Nights Nights
3 Off Off Off Days Days Days Days
4 Off Off Off Off Off Off Off

Five and two

The five and two provides 24/7 coverage using 4 crews and 12-hour shifts over a fortnight. Average hours is 42 per week but contains a 60-hour week which can be challenging.

Five and two shift example:

Week Fri Sat Sun Mon Tues Weds Thurs Fri Sat Sun Mon Tues Weds Thurs
1 Nights Nights Nights Off Off Nights Nights Off Off Off Days Days Off Off
2 Days Days Days Off Off Days Days Off Off Off Nights Nights Off Off
3 Off Off Off Swing Swing Off Off Swing Swing Swing Off Off Swing Swing
4 Off Off Off Days Days Off Off Days Days Days Off Off Days Days

Seven-day eight-hour rotating shift

The seven-day eight-hour rotating shift provided 24/7 coverage using 8-hour shifts with 14 crews. Consisted of a morning shift from 07:00 to 15:00, a swing shift from 15:00 to 22:30 and a night shift from 22:30 to 07:30 Each shift was worked for five days straight. The 8-hour shifts allowed vacations and absences to be covered by splitting shifts or working double shifts. The run of day shifts was 56 hours but the 8-hour shift provided time for some socializing after work. Was once common in the pulp and paper industry in the Western United States but has been largely replaced by a 8 days, 8 swing, 5 nights, 9 off, 8-hour rotation.

Seven-day eight-hour rotating shift example:

Week Sun Mon Tues Weds Thurs Fri Sat
1 Day Day Day Day Day Off Off
2 Swing Off Off Swing Swing Swing Swing
3 Night Night Off Off Night Night Night
4 Day Day Day Day Off Off Day
5 Swing Swing Swing Swing Swing Off Off
6 Off Night Night Night Night Night Off
7 Off Off Day Day Day Day Day
8 Swing Swing Swing Swing Swing Off Off
9 Night Off Off Night Night Night Night
10 Day Day Off Off Day Day Day
11 Swing Swing Swing Off Off Swing Swing
12 Night Night Night Night Off Off Night
13 Day Day Day Day Day Off Off
14 Off Swing Swing Swing Swing Swing Off
15 Off Off Night Night Night Night Night

Sixes

Submarine sailors in the American Navy engage in a pattern known as sixes while underway. Instead of a 24-hour day, the ship operates on an 18-hour schedule. Any given individual is scheduled to stand watch for 6 hours, perform any other duties and engage in leisure time for 6 hours, then sleep for 6 hours. If enough personnel are available, a given watchstation may benefit from a fourth man referred to as the midnight cowboy. He will stand the same 6-hour watch in a given 24-hour period, usually from midnight to 06:00 (hence the midnight portion of the name, which is most often shortened to just cowboy) and the person who would normally stand that watch is free. This gives rise to a schedule of six on, twelve off, six on, thirty off, six on, twelve off.

Firefighting schedules

In many departments, firefighters work 24-hour shifts. They are authorized to sleep in the fire station at night but are still subject to calls for service the entire shift. Departments have many options for scheduling firefighters for coverage. One option is 24 on/48 off, where a firefighter will work 24 hours and have 48 hours off, regardless of the day of the week or the holidays. Often they will be scheduled in an A–B–C pattern. Thus, a firefighter will be assigned to A, B or C shift and work whenever that letter is on the calendar.

California roll shift schedule

Another option is known as a California roll, where some shifts will be close together but allow for several days off. One option is thus:

Time Sat Sun Mon Tues Wed Thur Fri
Week 1 07:00 to 07:00 A B A C A C B
Week 2 07:00 to 07:00 C B

where a firefighter will work 24 hours on, 24 off, 24 on, 24 off, 24 on, 96 hours (4 days) off.

Another way to do this is:

Time Sat Sun Mon Tues Wed Thur Fri
Week 1 07:00 to 07:00 A B A B C A C
Week 2 07:00 to 07:00 A B C B C

where a firefighter will work one day, off one, work one, off two, work one, off four days.

Four-platoon 24-hour shift schedule

Another variation of the 24-hr shift schedule is a 4-platoon system, averaging 42 hours/week. Thus, the schedule is 24 on, 48 off, 24 on, 96 off, on a 4-day rotation.

Week Mon Tues Wed Thur Fri Sat Sun
Week 1 A D B A C B D
Week 2 C A D B A C B
Week 3 D C A D B A C
Week 4 B D C A D B A
Alternate Four-platoon 24-hour shift schedule

Another variation of the 24-hr shift schedule is a 4-platoon system, averaging 48 hours per week for three weeks and 24 hours the fourth week. Thus, the schedule is 24 on, 72 off.

Week Mon Tues Wed Thur Fri Sat Sun
Week 1 A B C D A B C
Week 2 D A B C D A B
Week 3 C D A B C D A
Week 4 B C D A B C D

Graveyard shift

Graveyard shift, night shift or third shift (3rd shift) means a shift of work running through the early hours of the morning, especially shifts from midnight until 08:00 or from 23:00 until 07:00. There is no certainty as to the origin of this phrase; according to Michael Quinion it is little more than "an evocative term for the night shift … when … your skin is clammy, there's sand behind your eyeballs, and the world is creepily silent, like the graveyard."[15]

In 2007, the World Health Organization (WHO) announced that working the graveyard shift would be listed as a "probable" cause of cancer.[16][17][18]

Industries

Industries requiring 24/7 cover are those that employ workers on a shift basis, for example:

See also

References

  1. ^ Sloan Work and Family Research, Boston College. "Shift work, Definition(s) of". http://wfnetwork.bc.edu/glossary_template.php?term=Shift%20work,%20Definition(s)%20of. Retrieved 2007-12-01. 
  2. ^ Institute for Work & Health, Ontario, Canada. "Fact Sheet, Shiftwork" (PDF). http://www.iwh.on.ca/system/files/documents/factsheet_shiftwork_2003.pdf. Retrieved 2007-12-01. "...outside regular daytime hours (i.e., between approximately 0700 and 1800 Monday through Friday)." 
  3. ^ Fido A, Ghali A. "Detrimental effects of variable work shifts on quality of sleep, general health and work performance" (Abstract). Med Princ Pract 2008;17(6):453-7. http://www.websciences.org/cftemplate/NAPS/archives/indiv.cfm?ID=20083138. 
  4. ^ Cohen M, Lippman M, Chabner B. Role of pineal gland in aetiology and treatment of breast cancer. Lancet 1978;2:14–16.
  5. ^ Schernhammer E, Schulmeister K. Melatonin and cancer risk: does light at night compromise physiologic cancer protection by lowering serum melatonin levels? Br J Cancer 2004;90:941–943.
  6. ^ Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology 2001; 12:74–77.
  7. ^ Hansen J. Light at night, shiftwork, and breast cancer risk. J Natl Cancer Inst 2001; 93:1513–1515.
  8. ^ Schernhammer E, Laden F, Speizer FE et al. Rotating night shifts and risk of breast cancer in women participating in the nurses' health study. J Natl Cancer Inst 2001; 93:1563–1568.
  9. ^ IARC Press release No. 180
  10. ^ WNPR, Connecticut Public Radio. "The health of night shift workers". Connecticut Public Radio, WNPR. http://www.publicbroadcasting.net/wnpr/news.newsmain?action=article&ARTICLE_ID=1191176&sectionID=760. Retrieved 2007-11-30. 
  11. ^ Erin Allday, Chronicle Staff Writer (24 March 2008). "Keeping the 'grave' out of 'graveyard shift'". San Francisco Chronicle. http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2008/03/23/MNGPVK33E.DTL. Retrieved 2009-11-19. 
  12. ^ Navara KJ, Nelson RJ (2007) The dark side of light at night: physiological, epidemiological, and ecological consequences. J. Pineal Res. 2007; 43:215–224
  13. ^ Santhi, N; Aeschbach D, Horowitz TS, Czeisler CA (2008). "The impact of sleep timing and bright light exposure on attentional impairment during night work" (abstract online). J Biol Rhythms 23 (4): 341–52. doi:10.1177/0748730408319863. PMC 2574505. PMID 18663241. http://www.websciences.org/cftemplate/NAPS/archives/indiv.cfm?ID=20082344. Retrieved 2008-08-05. 
  14. ^ Alan Blinder and William Baumol 1993, Economics: Principles and Policy, Harcourt Brace Jovanovich, San Diego, p. 687.
  15. ^ "Saved by the Bell", Worldwidewords.org, March 27, 2000
  16. ^ [1] An estimated twenty percent of the American labour force works the night shift.
  17. ^ Overnight shift to be classified as 'probable' cancer cause", CNN, November 29, 2007
  18. ^ http://www.thelancet.com/journals/lanonc/article/PIIS147020450770373X/fulltext

Further reading

External links

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