Light-dark cycles are popularly used as an entraining agent in order to study an organism's rhythm, as the length of the cycle can be easily manipulated within a laboratory. The naturally occurring light-dark cycle is approximately 24 hours in length, which corresponds to the length of circadian rhythms. Neurons are nerve cells that make up a large part of the nervous system. Pacemaker is an internal mechanism, which keeps time for the circadian clock by either coordinating or constructing rhythms. This suggests that the brain does not control all circadian rhythms.
Phase shift is the resetting of the internal clock due to an entraining agent.
A pulsatile is a regular rhythmic movement. The beating of a heart is pulsatile. Rhythm refers to a pattern of reoccurring events. In biology, rhythms can be loosely thought of as a cycle. An example would be a sleeping pattern.