Internal clock

Also known as the biological or physiological clock. The internal clock is the central element of the timing system. Its progress is genetically predetermined but is essentially controlled by the SCN and therefore influenced significantly by light. Light is consequently the most important “zeitgeber” for the internal clock; it synchronizes the internal clock with the external day. The status of the internal clock, in other words the phase of the biological clock, is the sum of overlaying all the body’s circadian rhythms. The phase can be largely deduced from the melatonin level curve.