By Mitchell Turner and Dr Ian Dunican

What we eat is essential for our health, but when we eat may be just as important. Circadian rhythms are traditionally thought only to regulate when we are awake or asleep; however, they also influence other behaviours, including eating. For example, circadian rhythms increase our hunger in the evening and promote greater energy expenditure after a morning meal ¹. Like sleep, we have windows of time when we should eat. Studies have shown that later sleep times are associated with increased caloric intake after dinner, increased fat intake and increased fast food consumption in adults and children ^{2, 3}. In addition, increased caloric intake later in the evening (after 8:00 pm) can raise basal metabolic index (BMI) and body fat percentage ^{2, 4}. Therefore, individuals should consider the timings of when they sleep and eat for effective weight loss or management.

Shift work is considered a severe desynchronisation of circadian rhythms as sleeping and eating timings are reversed. In addition, the misalignment of shift workers eating and sleeping windows can desynchronize their peripheral body clocks that regulate metabolism.

This desynchronisation has been shown to increase ghrelin (a hormone that stimulates hunger) and decrease leptin levels (a hormone that sends a signal that you are full) hormone, making us feel more hungry and less full ¹. Thus, shift workers may have an increased risk of obesity, type 2 diabetes, heart disease and metabolic syndrome ⁵.

Current lifestyle factors, such as work and social events, have resulted in potentially profound changes in the timings of eating and sleeping between weekdays and weekends. The difference in sleep-wake behaviour between weekdays and weekends is termed social jet lag and is a risk factor for metabolic health and obesity ⁶. In addition, a recent study has shown that more significant variability in meal timings between weekdays and weekends, termed ‘eating jet lag, significantly increases individuals’ BMI ⁷. These studies highlight the importance of keeping eating and sleeping windows consistent, despite changes in lifestyle factors.

The amount of sleep we get can also have a substantial impact on our metabolism. Specifically, chronic sleep loss impairs glucose tolerance and increases the risk of diabetes ⁸. A strong relationship exists between sleep duration and obesity, with sleep duration increasing the risk of obesity; however, obesity also decreases sleep duration by causing sleep disturbances ⁸. The impact of sleep duration on obesity, often measured by BMI, is thought to be due to its known influence on appetite regulation. Sleep deprivation decreases leptin levels, a hormone that makes us feel full and increases energy expenditure, and increases ghrelin levels, a hormone that stimulates appetite ⁸.

Current research evidence suggests that meal timings, as is the case with sleep timings, should be consistent. In addition, eating later in the evening before going to bed should be avoided. Finally, getting the recommended amount of sleep, seven to nine hours for adults ⁹, is vital for metabolic health.

References

1. Boege HL, Bhatti MZ and St-Onge MP. Circadian rhythms and meal timing: impact on energy balance and body weight. Curr Opin Biotechnol 2021; 70: 1-6. 20200929. DOI: 10.1016/j.copbio.2020.08.009.
2. Baron KG, Reid KJ, Kern AS, et al. Role of sleep timing in caloric intake and BMI. Obesity (Silver Spring) 2011; 19: 1374-1381. 20110428. DOI: 10.1038/oby.2011.100.
3. Spaeth AM, Hawley NL, Raynor HA, et al. Sleep, energy balance, and meal timing in school-aged children. Sleep Med 2019; 60: 139-144. 20190216. DOI: 10.1016/j.sleep.2019.02.003.
4. Thomas EA, Zaman A, Cornier MA, et al. Later Meal and Sleep Timing Predicts Higher Percent Body Fat. Nutrients 2020; 13 20201229. DOI: 10.3390/nu13010073.
5. Kervezee L, Kosmadopoulos A and Boivin DB. Metabolic and cardiovascular consequences of shift work: The role of circadian disruption and sleep disturbances. European Journal of Neuroscience 2020; 51: 396-412. DOI: 10.1111/ejn.14216.
6. Parsons MJ, Moffitt TE, Gregory AM, et al. Social jetlag, obesity and metabolic disorder: investigation in a cohort study. Int J Obes 2015; 39: 842-848. DOI: 10.1038/ijo.2014.201.
7. Zeron-Rugerio MF, Hernaez A, Porras-Loaiza AP, et al. Eating Jet Lag: A Marker of the Variability in Meal Timing and Its Association with Body Mass Index. Nutrients 2019; 11 20191206. DOI: 10.3390/nu11122980.
8. Van Cauter E, Spiegel K, Tasali E, et al. Metabolic consequences of sleep and sleep loss. Sleep Med 2008; 9: S23-S28. DOI: 10.1016/s1389-9457(08)70013-3.
9. Hirshkowitz M, Whiton K, Albert SM, et al. National Sleep Foundation’s sleep time duration recommendations: methodology and results summary. Sleep Health 2015; 1: 40-43. DOI: 10.1016/j.sleh.2014.12.010.