Switching from the day shift to working evenings or nights may affect your waistline, according to a new study from Australia, part of a growing body of research indicating working at night can lead to weight gain.
Nurses and midwives in the study who switched from working mostly during daylight hours to working odd hours in the evening and night saw an increase in their body mass indexes over a two-year period. In contrast, those who switched from working at night to working in the day had decreases in their BMIs.
The findings held even after the researchers took into account factors that could influence a person's weight, including diet quality, physical activity, smoking and alcohol consumption.
Isabella Zhao, of the University of Queensland School of Nursing and Midwifery in Brisbane, and colleagues surveyed more than 2,000 nurses and midwives in Australia and contacted them again two years later.
Participants were asked to report their normal work hours, as well as their height and weight, which researchers used to calculate their BMIs. (A person with a BMI between 25 and 30 is considered overweight, and a person with a BMI of 30 or higher is considered obese.)
Participants who switched from working evenings and nights to working during the day saw their BMIs decrease by 3 units on average.
In contrast, participants who continued to work the night shift, or who changed from working the day shift to working the night shift, saw an increase in their BMI of about 0.5 units, the researchers said.
Night shift workers may gain weight because their biological clocks are disrupted, Zhao said. Studies have shown that people who work at night produce less leptin, a hormone that signals the body to stop eating, and more cortisol, which has been linked to obesity, Zhao said. However, more research on this topic is needed, Zhao said.
Some factors that might have affected the results were not included in the study, such as whether participants suffered from depression, the researchers said. In addition, the researchers did not know at what point during the study the participants changed shifts.
The study is published in the May issue of the Journal of Occupational and Environmental Medicine.
Pass it on: Switching shifts from day to night may cause weight gain, while switching from night to day may lead to weight loss.