Rotating shifts and sleep: why your body clock cannot keep up, and what helps

Whether you can shift your body clock to fit your job depends on what kind of schedule you work. A steady run of the same nights gives your clock a stable target. A schedule that keeps moving does not. This article covers why, and which tools fit which.

Can you adjust your body clock to night shift?

Only partly, and less often than people hope. A review of six studies that tracked melatonin, the best-known marker of the body clock, in permanent night workers found that fewer than 3% showed complete adjustment, and fewer than one in four showed enough adjustment to benefit from it ^[1]. The author concluded that in ordinary environments, permanent night shifts are unlikely to produce enough adjustment to benefit health and safety for most people ^[1].

The American Academy of Sleep Medicine (AASM) guideline on shift work disorder, published so far as a draft for public comment and a conference abstract, describes the same general picture: the body clocks of the majority of shift workers do not adapt to their schedules, so most of its recommendations aim at managing sleepiness and insomnia rather than at shifting the clock ^[2].

What works on a fixed night schedule?

If you work the same nights week after week, your clock has a stable target. For people who want to shift toward their work hours, the AASM guideline suggests bright light during the night shift, fixed daytime sleep timing, and reduced-light glasses on the morning trip home ^{[2, 3]}. These are conditional recommendations based on very low certainty evidence, and the studies behind them achieved only partial adaptation ^[2].

The reason timing matters is well understood in the laboratory. In a study of 21 healthy volunteers, bright light given before the body's temperature minimum (the low point of the daily temperature cycle) pushed the clock later, light given after it pushed the clock earlier, and light centred on the low point itself did not shift it ^[4]. That is why a rule like "get bright light at 3 a.m." cannot be right for everyone: the effect depends on where your own clock sits.

In one field study of night-shift nurses, those who had bright light during their shifts and wore dark goggles on the commute home, while keeping stable eight-hour daytime sleep schedules, slept 7.1 hours on average versus 6.6 hours in the comparison group ^[5]. It is one study of modest size, so treat it as encouraging rather than settled.

What about rotating or changing shifts?

If your schedule keeps moving, such as days, then evenings, then nights, the AASM is blunt. For rotating or frequently changing shifts, it says, circadian adaptation is not a realistic goal of treatment ^{[2, 3]}. Your clock cannot chase a target that keeps jumping. This is not a failure on your part. There is no stable clock position to shift toward.

What the guideline does suggest for sleepiness is not about the clock, so it applies to rotating workers too: a nap before the night shift, caffeine, bright light during the shift, and a snack rather than a full meal during the night ^{[2, 3]}. All are conditional recommendations with low or very low certainty ^[2]. Those tools are covered in naps, caffeine and light.

Does the direction of rotation matter?

For people with shift work disorder and excessive sleepiness, the AASM suggests a clockwise rotation (moving later: morning shifts, then evenings, then nights) over a counterclockwise one. The certainty of the evidence is very low ^{[2, 3]}. The draft guideline did not find a reason to prefer 8-hour over 12-hour shifts, though it did not look at shifts longer than 12 hours ^[2].

Where is the evidence thin?

• Evidence for the recommendations comes largely from laboratory studies of simulated shift work in healthy adults and from field studies of shift workers who were not formally diagnosed, plus a small number of randomized trials in patients ^[2].
• Almost every AASM suggestion on this page rests on low or very low certainty evidence ^[2].
• Studies mainly measured immediate symptoms, not long-term health effects ^[2].

What to do if you rotate

Since clock-shifting is not on the table, the practical focus shifts to protecting as much sleep as you can and keeping some of it consistent. Anchor sleep is one idea for doing that, with its own limits. If the main problem is sleeping after a shift rather than staying awake during it, the guideline also suggests CBT-I (cognitive behavioural therapy for insomnia) ^{[2, 3]}. If sleepiness or insomnia lasts for months, see a doctor; shift work sleep disorder explains why.