Biological rhythms are endogenous; they persist even in the absence of environmental cues as they are driven by an internal mechanism, most notably the circadian clock.
Of the several possible cues (known as zeitgebers, German for 'time-givers') that can contribute to entrainment of the circadian clock, light has the greatest impact.
The phasic mode is when there is limited interaction with the environment to "reset" the clock every day by the amount equal to the "error", which is the difference between the environmental cycle and the organism's circadian rhythm.
Two properties, the free-running period of an organism, and the phase response curve, are the main pieces of information needed to investigate individual entrainment.
[8] When good sleep hygiene is insufficient, a person's lack of synchronization to night and day can have health consequences.