Shift work is like a bad Tinder date: it sounds dodgy, is rarely worth the hassle and there’s a risk it might kill you. Similarly, for some of us, it is a necessary evil. We have already discussed the impact of chrononutrition, and how optimal meal-timing and shiftwork tend to be mutually exclusive. Now, our focus shifts (lol solid pun) onto exercise and whether it can help mitigate or exacerbate the disruption to our circadian rhythm posed by shiftwork.

For the purposes of this article, circadian rhythm refers to the sleep-wake cycle. This is not the only example of circadian rhythm in the body.

Circadian Rhythm: Recap

Our natural “sleep-wake” cycle is an example of a circadian rhythm, and is estimated to be about 24 hours in length^[1]. This is autonomic (unconsciously regulated), and influenced by environmental cues called zeitbegers. The most influential zeitgeber is light. We react to changes in light (or lack thereof) in our environment^[2]. Other influencing factors include sunlight, exercise and food.

In humans, the timing of our sleep-wake cycle is marked by melatonin levels, core body temperature and cortisol levels. Melatonin is hypothesised to affect the sleep-wake cycle in two ways: shifting the circadian rhythm and promoting sleep onset^[3]. Melatonin is quite low during the light hours, sharply rises in the evening and continues to rise during the night. It returns to baseline around dawn^[4]. A decline in core-body temperature typically signals sleep onset^[5]. Finally, cortisol levels are naturally high in the morning and continue to decline throughout the day until the evening^[6].

Factors that negatively impact upon sleep-wake cycle synchrony are called chronodisruptors. Staying awake all through the biological night (AKA shiftwork) wreaks havoc on our natural light exposure and our normal melatonin and cortisol cycles. Shift work is well-documented in literature as having a disruptive effect on the natural circadian rhythms^[7],[8].  

 Nothing will disrupt your melatonin and cortisol cycles like staying awake all night on a shift.

Shiftwork & Chronodisruption

It is unfortunately well-established in literature that shiftwork is associated with a litany of negative health outcomes, such as cancer, increased risk of cardiovascular disruption and mental health disorders. This is owing to a disruption in the circadian rhythm^[9], and further exacerbated by the behavioural and environmental factors associated with shift work, such as the tendency to overeat during shifts, inappropriate meal timing and lack of physical activity in the daytime hours after a night shift.

How Exercise Affects Circadian Rhythm

The benefits of exercise in improving cardiovascular and metabolic health is well-documented in literature, and can be found through any Google Scholar search. Light is the most impactful mediator of the sleep-wake cycle, as discussed in the previous section. Exercise, and it’s timing can also influence the sleep-wake cycle.

Start With the Rats

Physical activity has been shown to influence the circadian rhythm in a number of rodent studies. One study found that mice placed on a running wheel (voluntary exercise) were better able to respond to disruption to synchrony of their circadian clock^[10]. Similarly, another study found wheel-running influenced the circadian rhythm by altering the core body temperature^[11].

So, all you have to do is hop on a spin bike and then you can control your natural sleep-wake cycle? Not quite. It is important to note that mice are nocturnal creatures, and their sleep-wake cycle is approximately 8 hours in length. This is obviously quite different to humans.

Human Research

As rats (of both human and rodent nature) are unreliable, we must turn to the human research before we form any hypothesis.

Exercise can help shift the phase in circadian rhythm in humans^[12]. What does this mean? If the phase shifts forward, bedtime and wake-up time are earlier in the day, and vice-versa.

Exercise Timing

Exercise can shift the circadian phase if performed at the right time of day. Literature tends to use melatonin onset as the marker. Temperature is prone to more variability and is therefore less reliable.

One study found that high-intensity exercise at night elevated melatonin (if melatonin was already elevated). Night exercise also resulted in delayed onset of melatonin secretion the following evening^[13]. A study of older adults concluded similar findings^[14]. A further study, using more controlled light-exposure (which is the most powerful zeitbeger as we have discussed!) concluded similarly, and found the delay to be most pronounced within four hours of initial melatonin onset^[15].

Chronotype

We must also consider chronotype when drawing practical applications from the research. Humans are loosely divided into two categories (this is obviously more intricate but for the purposes of this article it’ll do!): larks/morning people or owls/evening folk^[16]. Their circadian rhythms differ slightly. You can find out what type you are through several questionnaires that I’m sure you can Google.

Research has found that exercise at any time advanced the circadian rhythm for owl chronotypes. Conversely, morning exercise advanced the phase, but evening exercise delayed the phase for lark types^[17]. We must remember that chronotypes are classified through self-reporting, not objectively quantifiable measurements, so there are some caveats about drawing direct conclusions from the research.

Research versus Reality: Practical Applications

So, exercise can affect your circadian rhythm, and may be useful in offsetting the disruption caused by shiftwork. How can you apply this in your own life? As always, the research is often at odds with reality. Science is a neutral mistress, and does not consider the barriers of human life to its application.

Applying the Method to the Madness

Studies that found the most benefit to controlling “phases” often required the participants to carry out multiple bouts of exercise over the course of the night^[12]. Obviously, one cannot drop down and blast out a few burpees in the middle of a hospital. Therefore, the benefits may not be gotten if one cannot carry out the recommended intervention.

Not all exercise is created equal, and the majority of studies used aerobic or cardiovascular based interventions. I did not find much information on the direct correlation between resistance training and circadian rhythm other than the fact it tended to improve sleep quality regardless of the time of day it was carried out^[18].

One must also consider the personal preference of the individual. A certain training style may be more beneficial for sleep-wake cycle regulation, but if the individual does not like that method they are less likely to do it. Read: if you tell me I have to do regular cardio before a night-shift so I can sleep better the next night, it ain’t happening.

One must also consider the demands of a job. For example, if we are to conclude that larks should be up and exercising early on so that they can encourage their sleep-wake clock synchrony^[17], but they have been awake all night and busy, it is unlikely to happen. Chronotyping is all well and good, but after a busy night, the present need for sleep far outweighs the sleep quality of the future.

Conclusion

Exercise offers a whole host of benefits to the individual, and can influence the timing of our sleep-wake cycle (but not influence it’s length). It appears to be most influential on our circadian rhythm during the night time hours, but it is not the biggest influence (light is).

I don’t really think there is at present enough evidence to say what kind of exercise has the biggest impact and whether there is a “dose” (intensity/time etc.) threshold at which an effect is seen.

The timing of exercise is certainly impactful, but at the end of the day the overall benefits to our cardiovascular and metabolic health will be achieved regardless of what time of day/night it is carried out at. It’s the exercise equivalent of energy balance and meal timing. Meal-timing/chrononutrition can be influential, but energy balance is king.

I am sure I am wrong at least once during this article. Any corrections/feedback etc please get in touch.

References

1. Wirz-Justice, A. (2007) ‘How to Measure Circadian Rhythms in Humans’, Medicographia. Available at: http://www.chronobiology.ch/wp-content/uploads/publications/2007_02.pdf.
2. Honma, S. (2018) ‘The mammalian circadian system: a hierarchical multi-oscillator structure for generating circadian rhythm’, The Journal of Physiological Sciences. Available at: https://link.springer.com/article/10.1007/s12576-018-0597-5.
3. Khullar, A. (2012) ‘The Role of Melatonin in the Circadian Rhythm Sleep-Wake Cycle’, Psychiatric Times. Available at: https://www.psychiatrictimes.com/view/role-melatonin-circadian-rhythm-sleep-wake-cycle.
4. Benloucif, S., Guico, M.J., Reid, K.J., Wolfe, L.F., Zee, P.C. (2005) ‘Stability of Melatonin and Temperature as Circadian Phase Markers and Their Relation to Sleep Times in Humans’, Journal of Biological Rhythms. Available at: https://pubmed.ncbi.nlm.nih.gov/15834114/.
5. Kráuchi, K. (2006) ‘The human sleep-wake cycle reconsidered from a thermoregulatory point of view’, Physiology & Behaviour. Available at: http://www.chronobiology.ch/wp-content/uploads/publications/2007_05.pdf.
6. Adam, E.K., Quinn, M.E., Tavernier, R., McQuillan, M.T., Dahlke, K.A., Gilbert, K.E. (2017) ‘Diurnal Cortisol Slopes and Mental and Physical Health Outcomes: A Systematic Review and Meta-analysis’, Psychoneuroendocrinology. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5568897/.
7. Martínez Madrid, M., Campos, M., Pérez, J.M., Rol, M., Moreno-Casbas, T. (2013) ‘The challenge of chronodisruption assessment: the case of nursing staff shift workers’, Sleep Medicine. Available at: https://www.sciencedirect.com/science/article/abs/pii/S1389945713016717.
8. Cuesta, M., Boudreau, P., Dubeau-Laramée, G., Cermakian, N., Boivin, D.B. (2016) ‘Stimulated Night Shift Disrupts Circadian Rhythms of Immune Functions in Humans’, Journal of Immunology. Available at: https://www.jimmunol.org/content/196/6/2466.long?utm_source=TrendMD&utm_medium=cpc&utm_campaign=J_Immunol_TrendMD_0.
9. James, S.M., Honn, K.A., Gaddameedhi, S., Van Dongen, H.P. (2017) ‘Shift Work: Disrupted Circadian Rhythms and Sleep – Implications for Health and Well-Being’, Current Sleep Medicine Reports. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5647832/.
10. Leise, T.L., Harrington, M.E., Molyneux, P.C., Song, I., Queenan, H., Zimmerman, E., Lall, G.S., Biello, S.M. (2013) ‘Voluntary exercise can strengthen the circadian system in aged mice’, Age. Available at: https://pubmed.ncbi.nlm.nih.gov/23340916/.
11. Yasumoto, Y., Nakao, R., Oishi, K. (2015) ‘Free Access to a Running-Wheel Advances the Phase of Behavioural and Physiological Circadian Rhythms and Peripheral Molecular Clocks in Mice’, PLoS One. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0116476.
12. Youngstedt, S.D., Kline, C.E., Elliott, J.A., Zielinski, M.R., Devlin, T.M., Moore, T.A. (2016) ‘Circadian Phase-Shifting Effects of Bright Light, Exercise, and Bright Light + Exercise’, Journal of Circadian Rhythms. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4834751/.
13. Buxton, O.M., L’Hermite-Balériaux, M.L., Hirschfield, U., Cauter, E. (1997) ‘Acute and delayed effects of exercises on human melatonin secretion’, Journal of Biological Rhythms. Available at: https://pubmed.ncbi.nlm.nih.gov/9406031/.  
14. Baehr, E. K., Eastman, C.I., Revelle, W., Losee Olson, S.H., Wolfe, L.F., Zee, P.C. (2003) ‘Circadian phase-shifting effects of nocturnal exercise in older compared with young adults’, American Journal of Physiology. Available at: https://europepmc.org/article/med/12573982.
15. Barger, L.K., Wright, K.P., Hughes, R.J., Czeisler, C.A. (2004) ‘Daily exercise facilitates phase delays of circadian melatonin rhythm in very dim light’, American Journal of Physiology. Available at: https://pubmed.ncbi.nlm.nih.gov/15031136/.
16. Goel, N., Dinges, D.F. (2013) ‘Chronobiology: Biological Timing in Health and Disease’, Progress in Molecular Biology and Translational Science.
17. Thomas, J.M., Kern, P.A., Bush. H.M., McQuerry, K.J., Scott Black, W., Clasey, J.L., Pendergast, J.S. (2020) ‘Circadian rhythm phase shifts caused by timed exercise vary with chronotype’, Clinical Medicine. Available at: https://pubmed.ncbi.nlm.nih.gov/31895695/.
18. Dolezal, B.A., Neufed, E.V., Boland, D.M., Martin, J.L., Cooper, C.B. (2017) ‘Interrelationship between Sleep and Exercise: A Systematic Review’, Advances in Preventitive Medicine. Available at: https://www.hindawi.com/journals/apm/2017/1364387/.