昼夜节律（英语：circadian rhythm /sɜːˈkeɪdiən/），又译日夜节律、概日节律、日变周期、生理时钟，一种生理现象，以内源性、持续的，呈现以约24小时为周期的变动。包括植物、动物、真菌等，都被观察到有类似生理变化。英语：circadian，这个单字源自拉丁语：circa，为大约、大概的意思，拉丁语：diem，是一天、一日之意。合起来，在字面上的意思，是大约一天。
昼夜节律，由日变时钟（circadian clock）所驱动。
2017年来自美国的三位遗传学家杰弗理·霍尔（Jeffrey Hall）、迈克尔·罗斯巴希（Michael Rosbash）和迈克尔·扬（Michael Young）因发现控制昼夜节律的分子机制而共同赢得了诺贝尔生理医学奖。
延伸阅读
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