Biological Clock | Suprachiasmatic Nucleus | Circadian Rhythm

Biological Clock

Biological Clock - Definition

Biological clock is a 24-hour cycle in the physiological procedures.

Biological clock is also known as circadian rhythm.

The Terms On Which The Biological Clock Depends

Sunlight and the temperature are the external cues, on which the biological clock partly depends. On the whole, biological clock are defined by three criteria:

  • The clock persists in constant conditions (for example constant light) with a period of about 24 hours
  • The clock period can be reset by exposure to a light or dark pulse
  • The clock is temperature compensated, meaning that it proceeds at the same rate within a range of temperatures

Circadian Variations

Person’s biological clock is an internal clock that controls a variety of biological procedures according to an approximate 24-hour period. Most of a person’s body systems demonstrate circadian variations.

Sleep-wake cycle, the temperature regulation system, and the endocrine system are the body systems with the most prominent circadian variations.

Suprachiasmatic Nucleus

This clock, called the suprachiasmatic nucleus (SCN), is actually a pair of pinhead-sized brain structures that together contain about 20,000 neurons. The SCN rests in a part of the brain, just above the point where the optic nerves cross, which is called as called the hypothalamus.

Light that reaches photoreceptors in the retina creates signals that travel along the optic nerve to the suprachiasmatic nucleus. Signals from the suprachiasmatic nucleus travel to several brain regions, including the pineal gland, which responds to light-induced signals by switching off production of the hormone melatonin.


The body’s level of melatonin normally rises after darkness falls, making people feel sleepy. The suprachiasmatic nucleus also administers functions that are synchronized with the sleep/wake cycle, including body temperature, hormone secretion, urine production, and changes in blood pressure.

Jet Lag

Travelers suffer from disrupted biological clock, when they pass from one time zone to another time zone, which is known as jet lag. Some doctors try to manipulate the biological clock with a procedure called light therapy to reduce the effects of jet lag.

They expose people to special lights, many times brighter than ordinary household light, for several hours near the time the subjects want to wake up. This helps them reset their biological clocks and adjust to a new time zone.


All Article Categories