Study compares sleep patterns in homes with and without electric light

In an article published in Scientific Reports, researchers from Brazil, the UK and Sweden report alterations in sleep patterns and melatonin production in workers in Xapuri, Acre State, northern Brazil.

By Diego Freire

Agência FAPESP – Scientists have found significant differences in melatonin production and sleep duration between people living in homes with or without electricity and hence are exposed or not exposed, respectively, to artificial lighting.

The study was conducted by researchers from Brazil, the United Kingdom and Sweden to compare sleep patterns in a population of rubber tappers living and working in a remote area of the Amazon forest.

The initial research sample comprised 700 rubber tappers living in the Chico Mendes Amazon Extractive Reserve, located in Xapuri, Acre State, northern Brazil. The results of the study, published in Scientific Reports, an online journal owned by Nature Publishing Group, can be used to help evaluate the impact of work organization on worker health and wellbeing.

“Today, most people are exposed a great deal to artificial electric lighting in addition to the natural light cycle. This affects sleep duration and quality, and hence general health and wellbeing. Sleep restrictions have been associated with disorders such as obesity and type 2 diabetes, among others,” said Claudia Roberta de Castro Moreno, first author of the paper and a professor at the University of São Paulo’s School of Public Health (FSP-USP).

Moreno was responsible for the research project “The work temporal organization and its effects on health and wellbeing of factory workers and rubber tappers from an Amazonian extractive reserve,” supported by FAPESP. In the rubber tapper sleep study, she collaborated with researchers from the Federal University of Acre (UFAC), the Federal University of Paraná (UFPR), the Catholic University of Santos (UNISANTOS), the University of Surrey in England, and Stockholm University in Sweden.

According to Moreno, few studies have been performed to assess the impact of exposure to artificial light on sleep and health owing to the ubiquity of electricity in working people’s homes.

“Research on communities in a natural environment, without electricity, helps us understand how natural light-dark cycles influence circadian rhythm, the 24-hour body clock that tells us when to sleep and regulates many other physiological processes, particularly the part played by external factors,” she said.

The study of rubber tappers living on an extractive reserve also benefited from the biological homogeneity of this relatively isolated population, most of whom are descendants of the same ethnic group.

“It’s the first study ever done of a population of working people who not only have similar habits but also are ethnically homogeneous,” Moreno said. “Most of them are descendants of migrants from Brazil’s Northeast region and have lived in the area for generations. Hitherto, we’ve had only laboratory studies, with a lot of discussion about ‘entrainment’ of biological rhythms by intense lighting, but no comparisons in humans.”

According to Moreno, rubber tappers wake up early, usually around 4 a.m., and return from work in the forest well before nightfall. They go to sleep as soon as it gets dark because their homes do not have electricity. Therefore, they are exposed to 12 hours of light alternating with 12 hours of dark. “It’s like jumping back to a past way of life,” she said. “Almost everyone followed this kind of cycle a couple of centuries ago,” she said.

Results

The researchers found that electric lighting affected sleep duration in these workers. Rubber tappers with electricity in their homes slept 30 minutes less per workday than those without, “missing” 2.5 hours of sleep per week.

The study was conducted in two phases. In the first, participants were interviewed by a team from UFAC, who collected demographic and lifestyle data as well as information on each subject’s typical sleep pattern. In the second, three groups were selected according to whether they worked only during the day, only at night, or both.

Phase 2 entailed monitoring 20 rubber tappers from each group for a fortnight to record sleep patterns, melatonin timing and light exposure during work days and days off. Sleeping and waking times were collected by actigraphs, watch-like units worn on the non-dominant arm to measure motor activity, and compared with light exposure times captured by light sensors.

Saliva samples were collected and sent via São Paulo to the University of Surrey for analysis of melatonin, a hormone secreted by the pineal gland in the brain to help regulate the body’s circadian rhythm.

The analysis showed that dim light melatonin onset was significantly delayed in subjects with electricity compared with those without.

“Electric lighting delayed the start of sleep and reduced sleep duration during the work week,” Moreno said. “It also appears to interfere in the alignment of the circadian timing system with the natural light-dark cycle. On weekends, this leads to a ‘reboot effect,’ which helps workers sleep longer.”

Popularly known as the “biological clock”, the circadian cycle is an approximately 24-hour mechanism that controls and stabilizes the organism’s biochemical, physiological, and behavioral functions, including daily fluctuations in body temperature, brain activity, hormone secretion and heart rate.

“Modern society promotes a lifestyle that works against circadian alignment, with increased exposure to artificial light and use of electronic equipment. The study contributes to a better understanding of the right balance between artificial and natural light exposure and its influence on people’s health,” Moreno said.

The researchers are now studying the dietary patterns of the region’s workers, who eat a solid meal at 5 a.m. before starting the workday. “We believe the effects of this diet can also lead to significant differences in the functioning of the organism compared with people who only consume bread and coffee at daybreak, for example,” Moreno said.

The paper describing the results of the study was published in Scientific Reports and can be read at www.nature.com/articles/srep14074.