Mouse and human skin cells produce melanin on a 48-hour cycle -- ScienceDaily. They observed that a 48-hour cycle of exposure resulted in the darkest coloration of the cells while minimizing the effects of stress, even when they controlled for total dosage of exposure. "The results were so surprising," says Levy. "We expected daily synchronization of the cell's protective cycles." Levy and her colleagues, including co-senior author and systems biologist Shai Shen-Orr and his PhD student Avelet Alpert of the Technion -- Israel Institute of Technology, observed that MITF (microphthalmia-associated transcription factor) seemed to play a role in synchronizing the protective cycles. MITF was previously shown to control production of melanin and its spread to surrounding skin cells. They found that upon one ultraviolet exposure, MITF expression fluctuates every 48 hours. Another exposure 24 hours later seemed to disrupt this expression pattern.