The study analysed medical records and survey responses from 412 post-menopausal breast cancer patients with Oncotype DX.
Oncotype DX is a widely utilised test to guide treatment in early stage breast cancer by predicting likelihood of recurrence.
Patients were recruited at diagnosis and asked about the average sleep duration in the last two years.
Researchers, led by Cheryl Thompson, assistant professor at Case Western Reserve University School of Medicine, found that women who reported six hours or less of sleep per night on average before breast cancer diagnosis had increased Oncotype DX tumour recurrence scores.
"This is the first study to suggest that women who routinely sleep fewer hours may develop more aggressive breast cancers compared with women who sleep longer hours," Thompson said.
"We found a strong correlation between fewer hours of sleep per night and worse recurrence scores, specifically in post-menopausal breast cancer patients.
"This suggests that lack of sufficient sleep may cause more aggressive tumours, but more research will need to be done to verify this finding and understand the causes of this association," she said.
Researchers also found that the correlation of sleep duration and recurrence score was strong in post-menopausal women. The data suggested that sleep might affect carcinogenic pathway(s) specifically involved in the development of post-menopausal breast cancer.
"Short sleep duration is a public health hazard leading not only to obesity, diabetes and heart disease, but also cancer," said Li Li, the study co-author.
"Effective intervention to increase duration of sleep and improve quality of sleep could be an under-appreciated avenue for reducing the risk of developing more aggressive breast cancers and recurrence," Li said.
This study was published in Breast Cancer Research and Treatment.