Circadian Rhythm and Its Regulation

Regulation of circadian rhythm

1. Suprachiasmatic nucleus (SCN):
   The SCN, situated in the anterior hypothalamus above the optic chiasma, acts as the master biological clock of the body.
2. Light as the main synchronizer:
   Light is the most important external cue or zeitgeber. Light signals from the retina reach the SCN through the retinohypothalamic tract and synchronize the internal clock with the 24-hour light-dark cycle.
3. Molecular clock mechanism:
   In SCN cells, clock genes such as CLOCK, BMAL1, PER and CRY produce rhythmic changes in gene expression through a transcription-translation feedback loop. This generates an approximately 24-hour cycle.
4. Control of body functions:
   The SCN coordinates peripheral clocks in various organs by controlling autonomic, endocrine and behavioural functions. Thus it regulates sleep, body temperature, cortisol secretion and feeding pattern.
5. Melatonin secretion:
   The SCN regulates the pineal gland through sympathetic pathways. Darkness increases melatonin secretion, producing sleepiness; light suppresses melatonin secretion.
6. Other modifying factors:
   Food intake, exercise, social activity, stress and temperature can also influence circadian rhythm. Disturbance of these rhythms occurs in jet lag and shift work.