Write a short note on circadian rhythm. Add a note on melatonin.

Definition

The Biological Clock

• The master clock is the suprachiasmatic nucleus (SCN) of the hypothalamus
• It has an intrinsic period of about 24.2 hours, which must be reset daily
• Zeitgeber ("time giver") — the environmental cue that entrains it; light is the most powerful

Light → retinal ganglion cells containing melanopsin → Retinohypothalamic tract → suprachiasmatic nucleus → Paraventricular nucleus → cord → superior cervical ganglion → pineal gland → Light inhibits melatonin; darkness stimulates it

Melatonin

• Secreted by the pineal gland from serotonin, which comes from tryptophan
• Secreted in darkness, peaking between 2 and 4 a.m.; suppressed by light, especially blue light
• Functions — promotes sleep, signals day length (seasonality), antioxidant
• Secretion declines with age — one reason for poorer sleep in the elderly

RHYTHMS Under Circadian Control

Applied Aspects

• Jet lag — mismatch between internal clock and local time; eastward travel is worse, since advancing the clock is harder than delaying it. Melatonin and timed light exposure help
• Shift work → chronic desynchrony; associated with obesity, diabetes, cardiovascular disease and depression
• Timing of drugs matters — steroids are given in the morning to mimic the natural rhythm and minimise adrenal suppression
• Cardiovascular events cluster in the early morning, with the BP surge and peak platelet aggregability
• Blue light from screens at night suppresses melatonin and delays sleep onset