Lack of sleep increases the risk of aggressive breast cancers in women, particularly those who have hit menopause, a new study says.

Researchers found that not sleeping enough leads to more severe breast cancer and also increases the chances of relapse.

The study involved more than 400 post-menopausal women who were diagnosed with breast cancer. Researchers found that women who reported less than 6 hours of sleep per day had higher chances of recurring breast cancer. Breast cancer recurrence was predicted via Oncotype DX, a test that helps physicians decide the kind of treatment required in the initial stages of the cancer by calculating the chance of relapse.

"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. We found a strong correlation between fewer hours of sleep per night and worse recurrence scores, specifically in post-menopausal breast cancer patients," said Dr. Cheryl Thompson, from Case Western Reserve University School of Medicine and lead author of the study.

Researchers did not find a co-relation between pre-menopausal women and risk of aggressive breast cancer versus sleep duration suggesting that the mechanism of cancer development were different in women who had reached menopause.

"This suggests that lack of sufficient sleep may cause more aggressive tumors, but more research will need to be done to verify this finding and understand the causes of this association," Thompson added.

Lack of a good sleep has been associated with a number of other chronic conditions like stroke and cancer. Previous research published in the journal Obesity, says that short sleep duration was found to be independently associated with weight gain among all age groups.  Another study published in the American Journal of Epidemiology linked reduced sleep with modest weight gain in women.

"Short sleep duration is a public health hazard leading not only to obesity, diabetes and heart disease, but also cancer. 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," said Li Li, MD, PhD, a study co-author from Case Western Reserve University School of Medicine.