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Gary J. Schwartz is at the Fleischer Institute for Diabetes and Metabolism, and in the Departments of Medicine, of Neuroscience, and of Psychiatry and Behavioral Sciences, Albert Einstein College of Medicine, New York, New York 10461, USA.
Light has profound effects on human behaviour and physiology, from synchronizing sleep–wake cycles to inducing daily fluctuations in body temperature and energy metabolism. Our ability to see is mediated by a family of opsin proteins in the retina. When exposed to light, opsins modulate the flow of ions across neuronal membranes, ultimately activating the optic nerves1. In mammals, an opsin called opsin 5 (OPN5) is expressed in an unusual place — in neurons deep in the brain’s preoptic area2 (POA), which has a role in metabolism. Writing in Nature, Zhang et al.3 report a pathway by which OPN5 in the POA regulates heat production in mice. The authors’ findings open up the possibility of modulating metabolism by manipulating environmental light.