Female nurses working nights weigh [slightly] more than those working days

Body mass index scores are slightly higher among female nurses working night shifts (or a mix of day, evening and night shifts) than among those working regular day shifts, according to a recent study from the Institute for Work & Health. But we don’t yet know if this difference is important.

Published: April 23, 2013

Female nurses working night shifts or mixed shifts (i.e. days, evenings and nights) weighed just slightly more, based on body mass index (BMI) scores, than those working regular daytime schedules. This is according to a new study from the Institute for Work & Health (IWH) that looked at a 2005 “snapshot” of over 8,500 Canadian nurses.

Published in the February 2013 issue of Applied Nursing Research (vol. 26, no 1, pp. 24-31, doi:10.1016/j.apnr.2012.10.001), the study explored the relationship between shift work and BMI scores (a measure of body fat based on height and weight). From a public health perspective, this is a vital area of research, according to IWH Scientist Dr. Peter Smith, who led the study.

Some studies have reported that shift work is associated with higher rates of obesity, and obesity may be one of the pathways linking shift work to the increased risk of cancer and cardiovascular disease that we are seeing in the research recently, he says. However, not a lot is known about the relationship between shift work and obesity, and this is the area Smith set out to explore.

Survey included over 8,500 nurses

Smith and his research team turned to the National Survey on the Work and Health of Nurses (NSWHN), a 2005 Statistics Canada survey of almost 19,000 registered nurses from all 10 Canadian provinces and three territories. From among this sample, the researchers looked at 8,665 direct-care nurses who were working in hospitals or long-term care facilities and who were not working other jobs at the time.

The researchers calculated the BMI scores of these nurses (based on their self-reported height and weight) and compared them to their reported usual shift schedule: regular days, regular evenings, regular nights or mixed. They also looked at information available in the survey about working conditions (e.g. job strain and amount of respect and support at work), health behaviours (e.g. smoking and drinking alcohol) and employer-provided services (e.g. access to on- or off-site gym facilities and places to buy healthy food).

Small differences in BMI scores found

Smith and his team made these key findings:

  • After taking factors such as age, marital status, pain restrictions, mental health and more into account, shift schedules were associated with some slight differences in BMI scores. Namely, night and mixed shift schedules were associated with a small increase in BMI scores among female nurses. That is, female nurses working night shifts had BMI scores that were 0.67 points higher—and those working mixed shifts had BMI scores that were 0.44 points higher—than those working regular daytime schedules. According to the researchers’ calculations, this represented increased average weights of 1.66 kg (or 3.65 lb) and 1.11 kg (or 2.44 lb), respectively.
  • The relationship between shift work schedules and higher BMI scores was not affected by differing working conditions, employer-supported facilities or health behaviours—all of which are traditionally associated with BMI scores. In other words, if shift work schedules affected BMI scores, it did not appear that these factors made a difference.
  • Finally, no difference was found in the relationship between shift work and BMI scores for nurses who had access to fitness facilities or healthy eating options through work. That is, female nurses working night and mixed shift schedules who had access to employer-supported fitness and healthy eating services still had slightly higher BMI scores than their counterparts working regular daytime schedules.

Smith would like to see more research in this area. We first need to know if the difference in BMI scores among nurses working night shifts and mixed shifts is an important difference, he says. That is, does a difference of three-and-a-half pounds have potentially important health or social consequences?

As well, because this was a cross-sectional study—a snapshot of nurses at one point in time—the study did not provide information on how long nurses were working their reported usual work schedule. We don’t know the effect on BMI scores of working a particular type of shift over time, says Smith. Nor does the study allow us to explore the effect of different shift schedule patterns, such as rotating shifts, on BMI. Perhaps certain shift schedules lead to weight gain while others do not.