LOS ANGELES (FRONTLINE MEDICAL NEWS) – Current and previous night workers had significantly increased levels of hemoglobin A1c, compared with diurnal workers, preliminary results from an ongoing study showed. The finding sheds further insight into the link between environmental light, circadian rhythms, and metabolic disorders.

“To date, observational studies on bright light have revealed that evening bright light is associated with increased appetite and that bedroom light intensity is correlated with obesity,” Massimo Federici, MD , said at the World Congress on Insulin Resistance, Diabetes & Cardiovascular Disease. “It’s also been reported that artificial light is correlated with type 2 diabetes in the home setting and that daytime light exposure is positively correlated with body mass index. However, no studies have directly investigated the effect of acute light on human glucose metabolism.”

At the same time, observational studies of shift workers have shown that shift work is associated with metabolic disorders, but evidence for a causal relationship is limited, said Dr. Federici, professor of medicine and nutritional science at the University of Rome Tor Vergata. One study of night shift workers revealed reduced meal frequency but increased consumption of high energy snacks, physical activity, and altered sleep pattern, while a separate analysis found that permanent night shift workers showed only partial adaptation in 24-hour circadian rhythm of glucose and insulin levels ( Am J Physiol Endocrinol Metab. 2000;278[3]:E413-20 ).

Although few metabolic intervention studies using light have been done, Dr. Federici mentioned three of note. One, in patients with seasonal affective disorder and type 2 diabetes, showed reduced insulin requirements after light therapy ( Lancet. 1992;339[8800]:1065-6 ). Another, a short-term study of 25 obese subjects treated with 5,000 lux bright light therapy in addition to exercise, showed reduced body fat after 6 weeks ( Obesity 2007; 15[7]:1749-57 ). A third, in 34 obese subjects who were exposed to 1,300 lux bright light every morning for 3 weeks, showed a small but significant reduction in fat mass ( Obes Facts 2013;6:28-38 ).

As part of an ongoing project known as EuRhythDia , researchers including Dr. Federici set out to identify metabolic and molecular variables associated with shift work, and to test the effect of a lifestyle intervention that comprised light exposure, exercise, and melatonin. He presented unpublished results from one aspect of the trial: a cross-sectional analysis of 273 nurses divided into one of three groups: 64 diurnal workers (DW), 111 active night shift workers (aNW), and 98 prior night shift workers (pNW). Those with diabetes or taking oral antidiabetic drugs were excluded from the study.

The analysis showed that nurses in the pNW group were significantly older, at a mean of 39.7 years, than those in the DW group, whose mean age was 37 years, and the aNW group, who averaged 36.1 years. Those in the pNW group also had a significantly greater body mass index, compared with their counterparts in the aNW and DW groups (a mean of 25.7 kg/m2, vs. 24.8 and 23.7, respectively) as well has a higher mean waist circumference (a mean of 87.2 cm, vs. 84.6 cm and 82 cm).

The mean HbA1c was higher in the nurses with prior and active night shift work, at 5.3% each, than in the diurnal workers (5.1%, P less than .001).

When Pittsburgh Sleep Quality Index scores were used to evaluate sleep quality independent of work status, more than half of the study subjects (163) were classified as being “good sleepers,” while 110 were considered to be “bad sleepers.” Bad sleepers had a significantly higher mean HbA1c level compared with good sleepers (5.3% vs. 5.2%). Bad sleepers also had higher levels of HDL cholesterol (a mean of 60.8 mg/dL vs. 56.3 mg/dL).

Dr. Federici highlighted preliminary findings from a study of 32 aNW subjects who were assigned to treatment with warm light therapy at 1,000 lux for 30 minutes at 30 cm every morning for 3 months. They observed a mild improvement in the area under the curve of the oral glucose tolerance test at 24 weeks (12 weeks’ washout after 12 weeks of light therapy). “However, the effect was obtained not at the end of the intervention but at the end of the washout period,” he said.

He called for more studies going forward that take into account the effect of seasons as well as the effects of diet and exercise.

Dr. Federici disclosed that he receives editorial fees from Springer Nature group.


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