The circadian rhythm (time clock) of human body has to be respected in order to be in a good state of health. Repeated lack of sleep, working at night can lead to obesity, high blood pressure, heart disease, high blood sugar, type 2 diabetes and depression.

Researchers from Boston, Massachusetts, have conducted a study that enrolled two groups of women, for almost 20 years. The study, which appeared this December in PLoS Medicine, showed that women who worked rotating night shifts had greater risk of type 2 diabetes than women with regular hours. The risk increases in time, longer they have worked, greater the risk of developing type 2 diabetes.

“The association is quite strong and very consistent between the two cohorts. For nurses who spent a couple of years working rotating night shifts, there was a minimal increase in risk. But, for those with a very long duration of rotating shifts, the risk was almost 60 percent higher. This provides pretty strong evidence that the longer the rotating night shift work, the greater the risk of diabetes,” said Dr. Frank Hu, professor of nutrition and epidemiology at the Harvard School of Public Health in Boston.

In the study, rotating shift work means working three or more nights a month, plus days and evenings. An Pan, Eva S. Schernhammer, Qi Sun, Frank B. Hu conducted the U.S. Nurses’ Health Studies I and II. In NHS I 69,269 women aged 42–67 were enrolled, and 107,915 women aged 25–42 in NHS II. They had no diabetes, cardiovascular disease, or cancer and were followed for almost 20 years. In this period of time, 6,165 women in the NHS I and 3,961 women in the second group developed type 2 diabetes.

Women who did rotating shift work were compared with women who did not. Women who did one to two years of shift work had a 5 percent increase in type 2 diabetes, those who worked shifts for 3 to 9 years had a 20 percent, and nurses who worked for 10 to 19 years on rotating shifts had a 40 percent greater risk of type 2 diabetes. Women with more than 20 years on a rotating night shift had the highest risk, with a 58 percent increase in the risk of type 2 diabetes.

The medical scientists adjusted the data to account for body mass index and the linkage between shift work and type 2 diabetes was reduced. Overweight and especially obesity appeared to be greater risks for diabetes.

The researchers said “results suggest that an extended period of rotating night shift work is associated with a modestly increased risk of type 2 diabetes in women, which appears to be partly mediated through body weight. Proper screening and intervention strategies in rotating night shift workers are needed for prevention of diabetes“.

Nurses who did night shifts for over 20 years had a 58 percent risk of developing type 2 diabetes, a recently published study shows.

An association between varying or unusual work schedules and obesity and metabolic syndrome, high blood pressure and insulin resistance, was pointed out by some studies. Those factors are linked to an increased risk of type 2 diabetes.

Several studies on Japanese men showed a link between working the night shift and type 2 diabetes, but the linkage for men is still to be documented.

An Institute of Medicine report on breast cancer and environmental factors cited this week “growing evidence” that rotating shift work is “probably associated with increase risk for breast cancer“. They said more research is needed to establish the linkage and to find out what measures (aside from renouncing rotating shifts job) could lower these effects.

Why rotating shift work can increase the risk of type 2 diabetes and breast cancer?

Probably there are biological and behavioral reasons. Rotating shift work unbalance the body’s circadian rhythm (its natural time clock), the body’s ability to balance its need for energy is affected. This can lead to high blood sugar (hyperglycemia) and insulin resistance, both the key notes of type 2 diabetes.

Working on rotating shifts also disrupts eating and sleeping behaviors, and there was a tendency to smoke more between those who worked rotating shifts.

The hormonal balance (their secretion and their function) is disturbed by irregular sleep patterns, scientists think. Also “clock genes” are affected by lack of sleep or irregular sleep patterns, and this may set off cancerous cell growth in the breast tissue.

Obesity is a common factor in type 2 diabetes and in breast cancer, and shift workers appear to be at greater risk of weight gain, cause they eat at night.

“Shift work is an important risk factor for obesity and type 2 diabetes. This study increases the awareness of diabetes risk among people who work on a rotating shift, and the importance of diabetes screening, detection and prevention in this high risk group,” Dr. Frank Hu said.

However, more research is needed to confirm the findings, the scientists said.

Around 346 million persons have diabetes worldwide. Lots of them have type 2 diabetes, commonly caused by excess body weight, sedentary life, and lack of exercise. Untreated diabetes can damage eyes, arteries, heart, kidneys and nerves.

Apparently rotating shift work is becoming more common in an industrial society, thus is necessary to find a balance between work an rest, sleep and activity, to try to exercise at least two times a week, to have healthy meals and to see a general practitioner regularly.