Which hormone plays a primary role in the body's stress response, increases blood sugar, suppresses the immune system, and aids in the metabolism of protein, fat, and carbohydrate, while also showing diurnal variation that affects the sleep-wake cycle?

• A. Adrenaline
• B. Cortisol
• C. Insulin
• D. Thyroxine

Answer: (B)

Cortisol is the hormone that orchestrates many aspects of the body’s response to stress. It is released from the adrenal cortex as part of the HPA axis when we’re stressed, and it raises blood sugar by promoting gluconeogenesis and altering glucose uptake, providing quick and sustained energy. It also mobilizes amino acids and fats for energy use and has anti-inflammatory and immunosuppressive effects, which can dampen immune function during prolonged stress. A key feature is its diurnal rhythm: cortisol levels are highest in the early morning to help wake the body and gradually decline through the day, which ties into the sleep–wake cycle. The other hormones don’t fit all of these roles as cohesively—adrenaline drives rapid, short-term fight-or-flight responses; insulin lowers blood glucose and isn’t a primary stress hormone; thyroxine broadly regulates metabolism but isn’t principally tied to stress response or immune suppression or circadian sleep regulation.