Abstract
Metastasis is the most common event that determines survival in patients with breast cancer. The benefits of appropriate sleep in enhancing cancer patients' prognosis have been demonstrated. Likewise, emerging evidence has noted the positive impacts of regular circadian rhythm on cancer survival. Proper sleep and regular circadian rhythm can help to improve the cancer prognosis by enhancing the immune system. Besides, circadian rhythm disruption can assist cancer progression by promoting systemic inflammation. However, a recent study by Diamantopoulou et al. titled "The Metastatic Spread of Breast Cancer Accelerates during Sleep" demonstrated that sleep can aggravate breast cancer metastasis. This article outlines how the study design can affect this controversy.