Cancer related fatigue (CRF) is the most common cancer side effect and can severely interfere with activities of daily living long after completion of medical treatment. Pharmacologic agents to treat CRF have been studied but there is insufficient evidence to recommend their use. Non-pharmacological interventions for CRF have also been studied but are costly to implement and involve significant patient burden. This study investigates a novel low-cost/ low-burden intervention: systematic light exposure to treat CRF. Two hundred survivors of multiple myeloma and Diffuse Large B-cell Lymphoma between 1 month and 5 years post-autologous stem cell transplant (ASCT) will be recruited from two medical centers. The light will be administered by a small, personal Litebox daily for 4 weeks. Outcomes will be assessed at five separate time points, including baseline and follow-up. The study will specifically address recommendations made for interventions for CRF from the NCI Clinical Trials Planning meeting (JNCI, 2013).The proposed study will: 1) be the first large multisite study with a carefully delineated comparison condition to investigate the effects of light on CRF among ASCT survivors; 2) focus on a distinct, homogenous patient population; 3) include only survivors who experience clinical levels of CRF; and 4) address possible psychological and biological mechanisms. 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.