What is the term for a rhythm with a period of about 29 days that operates under constant conditions?

The term that describes a rhythm with a period of about 29 days, which operates under constant conditions, is known as a circalunar rhythm. This type of rhythm is closely tied to the lunar cycle and reflects the physiological and behavioral changes that occur in organisms in response to the phases of the moon.

Circadian rhythms, on the other hand, refer to cycles that last approximately 24 hours and are influenced mainly by daily light and dark cycles. Circannual rhythms are longer-term cycles that occur over the span of a year, often aligned with seasonal changes. While 'circannual cycle' may seem similar, it generally implies a longer time frame and is more associated with yearly cycles than specifically lunar influences. Therefore, the most accurate term for a 29-day rhythm is circalunar.