Chronobiology and Circadian Rhythms — MCQs

Question 1: Which hormone(s) exhibit(s) increased production during the day and decreased production at night?

• A. Adrenocorticotropic hormone (ACTH)
• B. Cortisol
• C. Insulin
• D. All of the above (Correct Answer)

Explanation: **Explanation:** The circadian rhythm is an internal biological clock regulated by the **Suprachiasmatic Nucleus (SCN)** of the hypothalamus. It coordinates the secretion of various hormones to align with the body’s metabolic needs during the activity (day) and rest (night) cycles. **1. ACTH and Cortisol (Options A & B):** The Hypothalamic-Pituitary-Adrenal (HPA) axis follows a distinct diurnal pattern. ACTH and Cortisol levels begin to rise in the early morning hours (around 3–4 AM), peaking just before or shortly after waking (the **Cortisol Awakening Response**). This prepares the body for daytime stress and activity by increasing blood glucose through gluconeogenesis. Conversely, levels reach their nadir (lowest point) around midnight. **2. Insulin (Option C):** Insulin secretion also exhibits a circadian rhythm, peaking during the day and decreasing at night. This is partly due to increased daytime food intake (postprandial spikes) and an inherent circadian regulation that enhances beta-cell sensitivity during the light phase to optimize glucose disposal. **Conclusion:** Since ACTH, Cortisol, and Insulin all show higher secretory activity during the day compared to the night, **Option D** is the correct answer. **High-Yield NEET-PG Pearls:** * **Melatonin:** The "hormone of darkness"; unlike the options above, it peaks at night (2–4 AM) and is inhibited by light. * **Growth Hormone (GH):** Secretion is pulsatile but shows a major peak during **Stage 3 (N3) Non-REM sleep**. * **Prolactin:** Levels also rise during sleep and peak in the early morning. * **Clinical Correlation:** Night shift workers or those with irregular sleep patterns often face "circadian misalignment," increasing the risk of metabolic syndrome and Type 2 Diabetes due to disrupted insulin and cortisol rhythms.