Silverin, B., Gwinner, E., Van't Hof, T. J., Schwabl, I., Fusani, L., Hau, M., & Helm, B. (2009). Persistent diel melatonin rhythmicity during the Arctic summer in free-living willow warblers. Hormones and Behavior, 56(1), 163-168. doi:10.1016/j.yhbeh.2009.04.002.