Shift work and sleep: how work shifts your biorhythm

Eight o'clock in the morning, the night shift is over. Outside the sun is shining, the neighbours are mowing the lawn, and you pull down the blinds. You are exhausted, fall asleep quickly – and are awake again after four hours. On your next day off, you no longer know when you are actually tired.

Or perhaps you don't work shifts at all: during the week the alarm goes off at six, at the weekend you sleep until ten. On Mondays you feel as if you have just come back from another time zone. Both have the same root: work sets a rhythm that does not match your internal clock.

Biorhythm: how your internal clock works

Your body has an internal clock that runs on roughly a 24-hour cycle. Experts call this the circadian rhythm; in everyday language people speak of the biorhythm. It controls when you get tired and when certain hormones rise and fall.

Two hormones are important pointers here. Melatonin, often called the sleep hormone, rises in the evening when it gets dark. Cortisol starts to rise in the early hours of the morning and reaches its peak shortly after waking up (PubMed 23148157). This is how the body gets ready for the day.

For the clock to run correctly, it needs a timekeeper. The most important one is light. Light in the evening can suppress melatonin and shift the internal clock later. In one study, people who read on a light-emitting e-reader in the evening fell asleep later, produced less melatonin, their internal clock ran later, and the next morning they were less alert than after reading a printed book (PubMed 25535358).

Shift work and your sleep rhythm: what changes

In industrialised countries, almost one in five employees works outside the usual daytime hours in some form (PubMed 28012806). If you work at night, you have to sleep when your internal clock is set to being awake, and be awake when it is set to sleep.

A large review of 38 meta-analyses and 24 systematic reviews shows that shift work leads to acute sleep loss, above all around night shifts and very early morning shifts (PubMed 27803010). Daytime sleep after a night shift is often shorter, and before an early shift the night gets short.

In some shift workers, this develops into a condition of its own: shift work disorder. It is a disorder of the sleep-wake rhythm with severe tiredness, insomnia or both as a result of shift work (PubMed 28012806).

Shift work: health effects according to studies

Studies show links between shift work and some diseases. What matters is how you read them: they are observations of large groups. They show a somewhat higher risk on average, but not that it affects everyone.

• Metabolism: Shift work is associated with a somewhat higher risk of type 2 diabetes and with weight gain (PubMed 27803010).
• Heart and blood vessels: A meta-analysis of around two million people found a higher risk of heart attacks. A higher risk of death was not found (PubMed 22835925).
• Accidents: Here too there is a link, similar to too little sleep (PubMed 27803010).

The authors of the large review stress that the individual studies had mixed results. Whether too little sleep is the actual route to these effects still needs to be researched (PubMed 27803010). This means: shift work is a reason to take good care of yourself, but not a reason for worry on its own.

Even without shifts: what social jetlag means

The term social jetlag comes from Munich chronobiologists. It describes the difference between sleep on working days and on days off. Many people, especially late types, sleep too little during the week and catch up at the weekend. The body then experiences a kind of time difference every Monday without having travelled (PubMed 16687322).

How large this difference is depends on your chronotype. This is your personal tendency to sleep early or late. In a large population study, greater social jetlag went hand in hand with a higher body mass index, even independently of sleep duration (PubMed 22578422). This is a link, not proof of a cause.

Screens, emails and commuting: how everyday life resets the clock

Even without night shifts, work can shift your biorhythm. The bright light from laptops and smartphones late in the evening pushes the internal clock later (PubMed 25535358). The next morning, the alarm still goes off at the same time. Long commutes shorten the time left for sleep, and you often miss out on daylight in the morning because you are sitting in the car, on the train or in the office.

Improving sleep with shift work: what you can try

The following points are general sleep hygiene, not a treatment:

• Darkness for daytime sleep. Blackout curtains or blinds, a sleep mask and earplugs.
• Use light deliberately. Light is the strongest timekeeper of the internal clock. Pay attention to when you are exposed to bright light, and discuss with your occupational health doctor what makes sense for your shift schedule.
• Caffeine early in the shift. In one study, caffeine still disturbed sleep when it was drunk six hours before going to bed (PubMed 24235903). For shift workers, this means the last coffee belongs in the first half of the shift.
• Fixed anchor times. A few hours of sleep that fall at the same time as often as possible give the internal clock orientation.
• Don't go to bed too late during the week. If you don't work shifts, you can reduce the difference between weekdays and the weekend.

Melatonin products and sleeping pills are not something to try out on your own. Discuss them with your doctor. Do not stop a prescribed medication without talking to your doctor first.

Sleep problems with shift work: when to see a doctor

A good first point of contact is your company doctor or occupational health service. They know shift schedules and can advise whether a different rhythm is possible. You should have the following checked by a doctor:

• severe tiredness that makes you almost fall asleep at the wheel or at machines
• sleep problems that do not improve even in weeks off
• loud snoring with pauses in breathing
• persistent low mood or exhaustion

Measure instead of guessing: what can be said about your rhythm

• The Cortisol daily profile measures cortisol in saliva in the morning, about 30 minutes after waking up, at midday and in the evening, plus DHEA from the morning sample. It shows the course over the day, not a night-time value.
• The Melatonin test measures free melatonin in a saliva sample taken at 2am in the dark. The amount varies greatly from person to person. The test does not determine your chronotype; that would require several samples in the evening. If you work night shifts, discuss beforehand which night makes sense for the sample.
• The Stress & sleep lab package combines both with further analyses of the stress axis.

The values are building blocks, not a diagnosis. The ORY expert team or your doctor can support you in interpreting them. Related reading: Waking up at 3am every night: what cortisol, alcohol and blood sugar have to do with it and Always tired even though your blood tests are normal?.

In brief

• The internal clock runs on roughly a 24-hour cycle, and light is its most important timekeeper.
• Melatonin rises in the evening, cortisol in the early morning.
• Shift work leads to sleep loss above all around night and early shifts; some people develop shift work disorder.
• Studies show links with metabolism, the heart and accidents, but the results are mixed.
• Social jetlag is the difference between sleep on working days and days off.
• Company doctors and occupational health services are good points of contact.

Frequently asked questions

Is shift work bad for your health?

The clearest effect is sleep loss around night and early shifts. Studies also show links with type 2 diabetes, weight gain, cardiovascular disease and accidents, with mixed results (PubMed 27803010).

What can I do about sleep problems caused by shift work?

Make sure it is dark for daytime sleep, drink caffeine early in the shift, look for fixed anchor times and talk to your company doctor. If the problems persist, shift work disorder should be checked by a doctor (PubMed 28012806).

What does social jetlag mean?

Social jetlag describes the difference between sleep on working days and on days off (PubMed 16687322). If you get up early during the week and sleep in at the weekend, you shift your internal clock back and forth every week.

How can I sleep better when working shifts?

A dark, quiet bedroom, the last coffee in the first half of the shift and sleep times that are as regular as possible are good starting points. Occupational health can clarify what suits your shift schedule.

What is the biorhythm?

The biorhythm is the body's internal clock. It runs on roughly a 24-hour cycle and controls tiredness, hunger and hormones such as melatonin and cortisol, among other things (PubMed 23148157).

Can a melatonin test tell me my chronotype?

No. The melatonin test measures a single sample at 2am. Determining your chronotype would require several samples in the evening. The value only shows the amount of melatonin at that point in time.

Sources

1. Chan S, Debono M. Replication of cortisol circadian rhythm. Ther Adv Endocrinol Metab. 2010. PubMed 23148157
2. Chang AM et al. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proc Natl Acad Sci U S A. 2015. PubMed 25535358
3. Wickwire EM et al. Shift Work and Shift Work Sleep Disorder: Clinical and Organizational Perspectives. Chest. 2017. PubMed 28012806
4. Kecklund G, Axelsson J. Health consequences of shift work and insufficient sleep. BMJ. 2016. PubMed 27803010
5. Vyas MV et al. Shift work and vascular events: systematic review and meta-analysis. BMJ. 2012. PubMed 22835925
6. Wittmann M et al. Social jetlag: misalignment of biological and social time. Chronobiol Int. 2006. PubMed 16687322
7. Roenneberg T et al. Social jetlag and obesity. Curr Biol. 2012. PubMed 22578422
8. Drake C et al. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med. 2013. PubMed 24235903