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•  Melatonin is secreted by the pineal gland to control circadian rhythms

Melatonin is a hormone produced by the pineal gland within the brain in response to changes in light

  • Light exposure to the retina is relayed via the suprachiasmatic nucleus (in the hypothalamus) and inhibits melatonin secretion
  • Melatonin is therefore secreted in response to periods of darkness, resulting in higher concentrations at night

Melatonin Secretion by the Pineal Gland

pineal gland

Circadian Rhythms

Melatonin secretion by the pineal gland of the brain plays a pivotal role in the control of circadian rhythms 

  • Circadian rhythms are the body’s physiological responses to the 24 hour day-night cycle
  • Circadian rhythms are driven by an internal (endogenous) circadian clock, although they can be modulated by external factors

Melatonin is the hormone responsible for synchronising circadian rhythms and regulates the body's sleep schedule

  • Melatonin secretion is suppressed by bright light (principally blue wavelengths) and hence levels increase during the night
  • Over a prolonged period, melatonin secretion becomes entrained to anticipate the onset of darkness and the approach of day 
  • Melatonin functions to promote activity in nocturnal animals and conversely promotes sleep in diurnal animals (like humans)
  • During sleep, necessary physiological changes occur in body temperature, brain wave activity and hormonal production
  • Melatonin levels naturally decrease with age, leading to changes in sleeping patterns in the elderly

Melatonin Secretion Over a Day-Night Cycle

circadian rhythm

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•  Causes of jet lag and use of melatonin to alleviate it

Jet Lag

Jet lag is a physiological condition resulting from a change to the body’s normal circadian rhythm

  • This alteration is caused by the body’s inability to rapidly adjust to a new time zone following extended air travel ('jet' lag)
  • The pineal gland continues to secrete melatonin according to the old time zone so that the sleep schedule is not synchronised to the new timezone

As a result of these sleep disturbances, individuals suffering from jet lag will often experience symptoms associated with fatigue

  • Symptoms of jet lag include headaches, lethargy, increased irritability and reduced cognitive function
  • Jet lag should only last a few days and symptoms should resolve as the body resynchronises its circadian rhythm

Some health professionals recommend taking melatonin near the sleep time of the new time zone to help recalibrate the body

  • By artificially increasing melatonin levels at the new night time, the body can respond quicker to the new day-night schedule

Time Zones Around the World

time zones