noun
diurnal rhythm
A biological rhythm with a period of about 24 hours; used in chronobiology and related fields.
日周リズムは、体内時計によって調節されている。
The diurnal rhythm is regulated by the biological clock.
概日リズム (circadian rhythm) is a near-synonym, but 日周リズム emphasizes the daily cycle more directly, while 概日リズム is the standard scientific term for endogenous ~24h rhythms.
Compound of 日周 (にっしゅう, 'daily cycle') and リズム ('rhythm'). 日周 is itself a Sino-Japanese word combining 日 ('day') and 周 ('cycle').