Severe fatigue, depression, sleep problems and cognitive impairment are the most commonly reported side effects of cancer treatment. These aversive side effects are hypothesized to be related to the disruption of circadian rhythms associated with cancer and its treatment. Exposure to Bright White Light (BWL) has been found to synchronize the circadian activity rhythms but research with cancer patients has been scarce. Therefore, the proposed randomized control trial (RCT) will test if systematic light exposure (sLE) will minimize overall levels of cancer-related fatigue (CRF), depression, sleep problems and cognitive impairment among breast cancer patients undergoing breast cancer treatment (i.e., surgery, chemotherapy). SLE incorporates the delivery of harmless UV-protected BWL or Dim White Light (DWL - standard comparison in light studies) delivered to patients by using special glasses for 30 minutes each morning, during their treatment. The proposed study, including a delineated comparison condition, will investigate the effects of BWL on CRF, sleep, depression, cognition, circadian rhythms, and inflammation markers among patients undergoing breast cancer treatment. The proposed RCT could have major public health relevance as it will determine if an easy-to-deliver, inexpensive, and low patient burden intervention reduces common side effects (e.g., CRF, depression, cognitive impairment) of cancer treatment (i.e., surgery, chemotherapy). Aim 1 - Assess whether Bright White Light (BWL) compared with Dim White Light (DWL) among breast cancer patients undergoing breast cancer treatment will minimize overall levels of CRF, depression, sleep problems, and cognitive impairment during and after breast cancer treatment, compared to healthy controls. Aim 2 - Determine whether the BWL intervention affects cortisol rhythms, circadian activity rhythms, melatonin rhythms, and inflammation markers that have been identified as correlates/causes of cancer-related side effects (e.g., CRF, depression, sleep problems). Aim 3 - Exploratory: Explore whether the effects of BWL compared to DWL on the cancer-related side effects (e.g., CRF, cognitive impairment) are mediated by the beneficial effects of the BWL in synchronizing circadian rhythms. Aim 4 - Exploratory: Explore potential moderators of the intervention including seasonality, chronobiology, personality, and social factors.