Which hormone shows diurnal variation and contributes to the sleep-wake cycle?

• A. Melatonin
• B. Cortisol
• C. Serotonin
• D. Dopamine

Answer: (B)

Cortisol follows a clear 24-hour rhythm that helps synchronize wakefulness with the day. Its levels rise in the early morning—the cortisol awakening response—to promote alertness, energy metabolism, and readiness for daytime activities, then gradually fall toward the night to support sleep. This diurnal variation is driven by the brain’s circadian clock and the hypothalamic-pituitary-adrenal axis, aligning our physiology with the light-dark cycle. While other hormones like melatonin rise at night to promote sleep, cortisol’s daytime peak and nighttime trough make it a key hormonal signal for waking and sustaining daytime alertness.