What is a bathyphase?

Answer 1

A bathyphase is a time period within a cycle during which the cycle is low, especially the low part of a sine wave fitted to a measurement of a circadian pattern or another biological rhythm.

Answer 2

A bathyphase is a term that refers to the period of time during which a deep-sea organism is active and feeding, typically at night when it ascends to shallower depths from the ocean floor to search for food. This behavior is common among many deep-sea creatures to avoid predators and take advantage of the cover of darkness to hunt.