Usingizi (kwa Kiingereza "sleep") ni hali ya binadamu na wanyama kupumzika kwa mwili na akili. Usingizi hutokea wakati mapigo ya moyo na upumuaji hupungua kwa ngazi zao za chini. Misuli yako inatulia na inaweza kuwa vigumu kukuamsha. Huwa hujitambui wala hujitawali sawasawa. Brown Ritchie E.; Basheer Radhika; McKenna James T.; Strecker Robert E.; McCarley Robert W. (2012). "Control of Sleep and Wakefulness". Physiological Reviews 92 (3): 1087–1187. doi:10.1152/physrev.00032.2011 . PMC 3621793 . PMID 22811426 . https://archive.org/details/sim_physiological-reviews_2012-07_92_3/page/1087. Parmeggiani, Pier Luigi, & Ricardo A. Velluti, eds. (2005). The Physiologic Nature of Sleep. London: Imperial College Press. ISBN 1-86094-557-0Script error: No such module "check isxn".. Parmeggiani, Pier Luigi (2011). Systemic Homeostasis and Poikilostasis in Sleep: Is REM Sleep a Physiological Paradox? London: Imperial College Press. ISBN 978-1-94916-572-2Script error: No such module "check isxn". Turek, Fred W. & Phyllis C. Zee, eds. (1999). Regulation of Sleep and Circadian Rhythms.