«MeSH» (på engelsk). National Library of Medicine and the National Institutes of Health. 2007. «Biological Clocks: The physiological mechanisms that govern the rhythmic occurrence of certain biochemical, physiological, and behavioral phenomena in plants and animals.»
«Annual Report of the Trans-NIH Sleep Research Coordinating Committee» (på engelsk). 2000. Henta 24. november 2007. «It now has been reported that the human circadian clock has a period of close to 24 hours (24 hr 11 min), similar to other species, rather than about 25 hours as previously thought.»