Entrainment

Entrainment is the process whereby two connected oscillating systems, having similar periods, fall into synchrony.

The system with the greater frequency slows down, and the other accelerates. Christian Huygens, a notable physicist, coined the term entrainment after he noticed, in 1665, that two pendulum clocks had moved into the same swinging rhythm, and subsequent experiments duplicated this process. The accepted explanation for this is that small amounts of energy are transferred between the two systems when they are out of phase in such a way as to produce negative feedback. As they become more in phase, the amounts of energy gradually reduce to zero. In the realm of physics, entrainment appears to be related to resonance.

An example from human biology is the tendency of the menstrual cycles of women living together to coincide. In this case it is possibly pheromones that are transferred rather than energy.

In the study of sleep it refers to one's actual sleep schedule matching his circadian rhythm, an aid to proper sleep hygiene.