Cancer related fatigue (CRF) is a stressful and constant tiredness related to cancer and/or its treatment. CRF is the most intense during treatment and can severely interfere with activities of daily living, such as tasks that require physical strength or thinking clearly. Prevalence of CRF has been reported to be as high as 94% during chemotherapy and as high as 34% five years after completion of treatment (Rotonda et al. 2013; Minton & Stone 2008). There is currently no generally-accepted treatment for CRF. However, there is evidence to suggest that light therapy can help with CRF. Non-pharmacological interventions for CRF have also been studied but are costly to implement and involve significant patient burden, particularly among those in active treatment. Given the clinical impact of CRF, the goal of this project is to investigate a novel, low-cost and low-burden intervention for Breast Cancer patients using a particular kind of light treatment called systematic light exposure (sLE) to treat CRF. Two hundred forty-eight breast cancer (BC) patients undergoing adjuvant or neoadjuvant chemotherapy will be recruited from Mount Sinai Medical Center and Memorial Sloan Kettering Cancer Center. The light will be administered by a small, personal Litebox daily throughout entire duration of chemotherapy. Outcomes will be assessed at eight timepoints during chemo, and a series of follow up assessments at 1 week, 1-month, 3-months and 6-months post-chemotherapy. This study will have major public health relevance as it will determine if an easy-to-deliver, inexpensive, and low patient burden intervention effectively reduces CRF or prevents it from worsening during chemotherapy. Specific Aims: Aim 1: Determine if sLE prevents CRF from worsening in BC patients undergoing chemotherapy Aim 2: Determine whether sLE affects sleep, depression and circadian activity rhythms. Exploratory Aim 3: Investigate sLE normalizes circadian cortisol rhythms. Exploratory Aim 4: Examine whether the effects of sLE on fatigue are moderated/mediated by sleep quality, depression, and/or circadian rhythms.