Which hormone, secreted by the pineal gland, is primarily responsible for regulating circadian rhythms?

Appeared in: HSSC Staff Nurse - 2015

Explanation

Melatonin is a hormone secreted by the pineal gland, a small endocrine gland in the brain.
Its primary function is the regulation of the circadian rhythm, which is the body's natural 24-hour sleep-wake cycle.
Melatonin synthesis and release are stimulated by darkness and suppressed by light, signaling to the body that it is time to sleep.
This light-dependent mechanism helps entrain the body's internal clock to the external environment.

Option A: Thymosin is secreted by the thymus gland, not the pineal gland. Its primary role is in the immune system, specifically in the maturation of T-lymphocytes.
Option C: Calcitonin is secreted by the thyroid gland. Its function is to regulate blood calcium levels, not circadian rhythms.
Option D: Aldosterone is a steroid hormone produced by the adrenal cortex. It regulates salt and water balance, which affects blood pressure, but it does not control the sleep-wake cycle.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Hormonal regulation of circadian rhythms as background academic context rather than a clinical decision trigger.
Melatonin supplements are commonly used to treat sleep disorders like insomnia and jet lag. Nurses often educate patients on their proper use.
Patient teaching on sleep hygiene is crucial. This includes advising patients to minimize exposure to blue light from screens before bedtime, as it can suppress melatonin production and disrupt sleep.
What if a patient works night shifts? Their natural circadian rhythm is disrupted. This can lead to shift work sleep disorder. Management may involve controlled light exposure, scheduled sleep, and sometimes, judicious use of melatonin supplements to help reset their sleep-wake cycle.

How to Approach the Question

First, identify the key terms in the question: 'hormone', 'pineal gland', and 'circadian rhythms'.
This is a factual recall question. Access your knowledge of the endocrine system.
Think specifically about the function of the pineal gland. Recall that its primary hormone is melatonin.
Connect the function of melatonin to the concept of 'circadian rhythms' or the sleep-wake cycle.
Evaluate the other options to confirm they are incorrect. Recall the source gland and primary function of Thymosin (thymus, immunity), Calcitonin (thyroid, calcium regulation), and Aldosterone (adrenal cortex, salt/water balance).
Select the option that correctly links the pineal gland and circadian rhythm regulation.

Concept Tested & Keywords

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E6 Physiology Ganong 27e Vol 1 Part 2 p. 59-61

E6 Physiology Essentials Sembulingam 10e Part 2 pp. 88-90, 89-91

Practise the full HSSC Staff Nurse - 2015

Attempt every question from this paper in a timed mock, then review the full solution for each one.