Circadian rhythms describe the body’s normal 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 a hormone produced by the pineal gland within the brain and is responsible for synchronising circadian rhythms and regulating the body's sleep schedule

• Melatonin is secreted in response to periods of darkness, resulting in higher concentrations at night

• Light exposure to the retina is relayed to the hypothalamus, which then inhibits melatonin secretion

  • The hypothalamus contains a small region called the suprachiasmatic nucleus (SCN) which acts as a circadian pacemaker

  • Exposure to blue wavelengths of light (higher during the day) will trigger the suppression of melatonin secretion

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