There is a growing body of evidence that exposure to green space is good for our health but a new study from the University of Adelaide has found that this may equally be due to how much light we are exposed to at night.

Spending time in green space can improve depressive symptoms, obesity, and sleep problems, and reduce the risk of breast and prostate cancer. Conversely, exposure to light at night, particularly urban light pollution, increases the risk of breast and prostate cancer, and can worsen depression, obesity and sleep problems.

Researchers identified a negative correlation between green space diversity and outdoor artificial light at night for Australian major cities—in other words, the greener your environment, the less the light pollution, and vice versa.

This makes intuitive sense, because the more developed an area is, the fewer trees there will be and the more lights there will be.

Published in Environmental Research, the study questions whether the health benefits of green space exposure may in part be a result of avoiding light at night.

"There seems to be a pattern here—yet, amazingly, no one has put these two things together—until now,"' said lead author Dr. Jessica Stanhope from the University of Adelaide's School of Allied Health Science and Practice.

"It is possible that these factors have been confounding each other in epidemiological studies of the associations between residential green spaces and improved health, and urban outdoor artificial light at night exposure and poor health.

"We have shown that green space is inversely associated with outdoor artificial light at night, making it unclear whether health outcomes result from the green space, the light at night, or possibly in an interaction of the two."

Researchers recommend that epidemiological studies focus on resolving this problem as a priority, so that recommendations can be made for interventions that would improve the public health. For example, to improve population health, is it more important to plant green space in urban areas to give people in cities better green space exposure, or is it better to invest that effort in reducing urban light pollution, or both?

"Some great studies have been done on the association between green space and health, which is a rapidly growing research area; and there are also very neat epidemiological studies of the adverse health effects of exposure to light at night," said Dr. Stanhope.

"It is now really important that future studies include both factors so that we can better understand their association—only then can we make better public health recommendations about planning health-giving sustainable urban landscapes."