Melatonin has a long history of studies which confirm its ability to inhibit cancer growth. Melatonin is present in high concentrations in the mitochondria of normal cells but is likely absent from the mitochondria of cancer cells, at least when isolated from tumors harvested during the day. Herein, we hypothesize that melatonin’s absence from cancer cell mitochondria prevents these organelles from metabolizing pyruvate to acetyl coenzyme A (acetyl-CoA) due to suppression of the activity of the enzyme pyruvate dehydrogenase complex (PDC), the enzyme that catalyzes the conversion of pyruvate to acetyl-CoA. This causes cancer cells to metabolize glucose to lactate in the cytosol (the Warburg effect). Since cancer cell mitochondria can take up nighttime pineal-derived melatonin from the blood, the indoleamine predictably promotes the conversion of pyruvate to acetyl-CoA in the mitochondria during the night. Thus, while cancer cells exhibit a typical cancer phenotype during the day, at night cancer cells have a more normal cell phenotype. Via similar actions, melatonin probably overcomes the insensitivity of cancers to chemotherapies. Hopefully, the hypothetical processes proposed herein will soon be experimentally tested.