Working the night shift isn’t just bad for sleep — it could be seriously bad for your health.

A study published in Cancer Epidemiology, Biomarkers & Prevention found that working during the wee hours over the long term was associated with a 19% greater risk of cancer among women.

Researchers also took a close look at female nurses who work night shifts and found they are at higher risk for six different forms of cancer. The findings showed these nurses had a 58% higher risk of breast cancer, a greater increase than any other job classification included in the study.

In addition, night-shift nurses had a 35% greater risk of gastrointestinal cancer and a 28% higher risk of lung cancer than people who didn’t work nights.

Author Xuelei Ma, M.D., said nurses might be more apt to get cancer screenings given their medical background.

“Another possible explanation for the increased cancer risk in this population may relate to the job requirements of night-shift nursing, such as more intensive shifts,” added Ma, an oncologist in the State Key Laboratory of Biotherapy and Cancer Center at West China Medical Center of Sichuan University.

“These results might help establish and implement effective measures to protect female night-shifters,” he said. “Long-term night-shift workers should have regular physical examinations and cancer screenings.”