Exercise, energy balance and the shift worker

Abstract

Shift work is now common in society and is not restricted to heavy industry or emergency services, but is increasingly found amongst 'white collar' occupations and the growing number of service industries. Participation in shift work is associated with increased body mass index, prevalence of obesity and other health problems. We review the behavioural and biological disturbances that occur during shift work and discuss their impact on leisure-time physical activity and energy balance. Shift work generally decreases opportunities for physical activity and participation in sports. For those shift workers who are able to exercise, subjective and biological responses can be altered if the exercise is taken at unusual times of day and/or if the shift worker is sleep deprived. These altered responses may in turn impact on the longer-term adherence to an exercise programme. The favourable effects of exercise on body mass control and sleep quality in shift workers have not been confirmed. Similarly, recent reports of relationships between sleep duration and obesity have not been examined in a shift work context. There is no evidence that exercise can mediate certain circadian rhythm characteristics (e.g. amplitude or timing) for improved tolerance to shift work. Total energy intake and meal composition do not seem to be affected by participation in shift work. Meal frequency is generally reduced but snacking is increased on the night shift. Unavailability of preferred foods in the workplace, a lack of time, and a reduced desire to eat at night explain these findings. 'Normal' eating habits with the family are also disrupted. The metabolic responses to food are also altered by shift work-mediated disruptions to sleep and circadian rhythms. Whether any interactions on human metabolism exist between timing or content of food intake and physical activity during shift work is not known at present. There are very few randomized controlled studies on the efficacy of physical activity or dietary interventions during shift work. Some favourable effects of such interventions on fatigue levels at work have been reported, but biological and behavioural outcomes relevant to long-term health and energy balance have not been studied adequately. In addition, recruitment and retention of research participants for randomized controlled trials of physical activity or dietary interventions have been very difficult. We present a model of the various behavioural and biological factors relevant to exercise and energy balance during shift work as a framework for future research.

Figure 1

Circadian rhythms of ingestion of total energy, proteins, fats and carbohydrates during three work shifts (morning, afternoon and night). Total energy intake did not vary between the shifts, but the timing of energy intake and macronutrients differed, e.g. morning shift workers consumed more energy and carbohydrate in the morning hours than other shifts. A cubic spline model was used to smooth the curves (De Assis et al. [25]). Permission pending.

Figure 2

The effects of a physical activity intervention on fatigue/alertness of female nurses working different shifts (morning, evening, night). The higher the number, the more alert the shift worker was. T - data for an exercise training intervention group. C - a control group which did not undertake the exercise intervention. * - P<0.05. ** - P<0.01. ⌐¬ - significant difference intervention and control groups in terms of mean pre-post changes. Harma et al. [89] Permission pending.

Figure 3

Schematic presenting the various behavioural and biological factors associated with energy expenditure and energy intake, and therefore energy balance, during shift work. Both energy expenditure and intake can be influenced by behavioural and biological factors, which act in combination to affect energy balance.

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