A U.S. agency tasked with studying carcinogens says working at night — specifically, exposure to light during routine night-time shifts — is linked to increased cancer risk.

The Department of Health and Human Services’ National Toxicology Program has preliminarily recommended adding light-at-night to its list of substances “known to be human carcinogens,” potentially putting it alongside other dangers such as arsenic, plutonium and tobacco.

After a two-year review of existing studies from around the globe, government researchers found the strongest link between light at night and increased breast cancer rates.

The extensive analysis looked at overnight light’s impact on circadian rhythm, connected hormonal changes and other behaviors or conditions common in shift workers.

“Few night shift workers are able to adapt their circadian rhythms to their altered sleep-work cycle, and women with more persistent shift work may have health problems,” report authors noted.

Preliminary conclusions will be peer-reviewed and will be the focus of a public meeting in October.