Cortisol and shift work: what night shifts do to cortisol, and how to test it

Shift work pulls cortisol out of its usual daily pattern. Cortisol normally peaks soon after you wake in the morning and falls to its lowest point around midnight, and sudden changes to when you sleep, such as night shifts or jet lag, activate the body's stress system and change that pattern.¹ In a hair study, younger shift workers had markedly higher long-term cortisol than day workers, along with a higher body mass index (BMI).² Shift work also makes most cortisol tests hard to use, because they are timed for people who sleep at night: NICE advises GPs to get specialist advice before testing shift workers.³ A hair test has no such problem. It does not change with the time of day, which makes it the most practical and accurate way to measure long-term cortisol from a single sample on any rota.⁴

How shift work disrupts the cortisol rhythm

In people who sleep at night, cortisol starts rising from about 4 in the morning, peaks within three hours of waking and then falls through the day.¹ In a healthy reference group, blood cortisol peaked at about 8.30 in the morning and stayed below its daily average from about 7.45 in the evening to 5.30 the next morning.⁵ Falling asleep itself dampens the system that drives cortisol.¹ See the daily cortisol rhythm for how the normal pattern works.

Working nights or rotating shifts breaks that link between sleep, light and the clock. Sudden changes to the sleep-wake cycle, such as shift work or jet lag, activate the HPA axis, the hormonal system that controls cortisol, and change the usual daily pattern.¹ Lost sleep adds to it. In a laboratory study, after a night of partial or total sleep deprivation, cortisol the following evening was 37 and 45 per cent higher, and the evening quiet period started at least an hour later.⁶ Read more in cortisol and sleep.

What hair studies show in shift workers

Because a single blood or saliva sample is so hard to interpret on a shifted day, hair has been used to ask whether shift work changes long-term cortisol. In a study of 33 shift workers and 89 day workers, the shift workers had higher hair cortisol: 47.32 against 29.72 pg/mg.² The difference was found only in the younger half of the group, where shift workers had 48.53 pg/mg against 26.42 pg/mg in day workers of a similar age. The younger shift workers also had a higher BMI, 27.2 against 23.7, and hair cortisol and BMI were positively related.² The authors suggested that higher cortisol and BMI may help explain the raised cardiovascular risk seen in shift workers.²

That study was small and measured each person once, so it cannot show that shifts caused the higher cortisol, or that cortisol caused the weight difference. Across many studies, the link between hair cortisol and BMI is consistent but weak, with a pooled correlation of r = 0.10.⁷ Sleep, meal timing and activity all differ on shifts too. See cortisol and weight for more.

Why blood and saliva tests are hard to time on shifts

Every standard cortisol test assumes a normal day. When a doctor checks for low cortisol, NICE advises a blood sample at 8 to 9 in the morning, when cortisol should be near its peak.⁸ One NHS laboratory states that a random blood cortisol taken at another time cannot be interpreted.⁹ For a night worker, 9 in the morning may be the end of the working day rather than the start, so NICE's guidance for GPs advises seeking specialist advice before testing shift workers, because it is uncertain when to test and the results can be hard to interpret.³

The late-night saliva test, a first-line test for Cushing's syndrome, depends on cortisol being at its lowest around bedtime. The Endocrine Society says it may not be appropriate for shift workers or people with variable bedtimes, and a 2021 international consensus advises avoiding it in night-shift workers.¹⁰¹¹ A 24-hour urine collection is less tied to the clock, because it gathers a whole day's cortisol, but it varies a lot from one collection to the next: about 52 per cent within the same patient in one large study, which is why two or three collections are advised.¹²¹¹ If you work shifts and a doctor suggests a cortisol test, tell them your pattern. Our guides to the blood cortisol test and the saliva cortisol test cover how each is done.

Why hair is unaffected by time of day

A hair sample holds the cortisol laid down as the hair grew. Scalp hair grows about 1 cm a month, so the 3 cm nearest the scalp reflect roughly the past three months.¹³ In two experiments with 94 people in all, hair cortisol was unrelated to the time of day the sample was taken and did not change after heavy sweating; the authors concluded that hair cortisol shows no daily variation.⁴ It does not matter whether you cut the sample after a night shift or on a day off.

Hair is also consistent. In a study of adults who gave saliva three times a day for 30 days, hair cortisol tracked their 30-day saliva average (r = 0.61), and the same people's hair results correlated at r = 0.84 from one month to the next, against 0.38 to 0.61 week to week for saliva.¹⁴ For a shift worker, whose single saliva or blood samples are harder to interpret than most people's, that consistency matters more than usual.

There is one thing hair cannot do. Because it averages cortisol over months, it cannot show when in the day your cortisol peaks, or whether your rhythm has shifted or flattened. Mapping the daily pattern needs several timed blood or saliva samples, which a specialist would arrange if there were a medical reason.

What helps: sleep first

The clearest evidence for shift workers concerns sleep. In a randomised trial of 74 nurses with shift work sleep disorder, a digital course of cognitive behavioural therapy for insomnia (CBT-I) reduced insomnia much more than online information about shift work did, a large effect (d = 1.11). In the therapy group, hair cortisol fell by 1.8 pg/mg, or 44 per cent from the starting level.¹⁵ That fall was measured within the therapy group rather than against the comparison group, so it is encouraging rather than conclusive.

The trial did not find effects on stress or depression scores, and hair cortisol in general does not track how stressed people say they feel.¹⁵¹⁶ So a lower result does not mean you will feel less stressed, and a higher one does not prove that your shifts are harming you. Our guides to how to lower cortisol and cortisol and stress cover the other approaches that have been tested.

What a hair test can and cannot tell a shift worker

A hair test can show your average cortisol over roughly the past three months, measured the same way whatever your rota. It can give a baseline before a change, such as moving off nights, starting treatment for insomnia or changing your sleep routine, and a second test three to six months later shows whether your levels have moved. Compare results only with your own laboratory's range, because ranges differ more than tenfold between methods: 0.84 to 8.07 pg/mg in a recent LC-MS/MS study against 17.7 to 153.2 pg/mg in an early immunoassay study.¹⁷¹⁸

It cannot diagnose shift work sleep disorder, Cushing's syndrome or Addison's disease, or prove that your shifts caused a result. If you have symptoms that could mean an adrenal condition, such as unexplained weight change, extreme tiredness, dizziness on standing or easy bruising, see your GP, tell them about your shifts, and let them choose the tests. See high cortisol symptoms and getting a cortisol test from your doctor.