What is the primary role of endorphins in the human body? 

• A. Regulate the circadian rhythm when it comes to our sleep-wake cycles.  
• B. Stimulate muscle contraction in voluntary movements.  
• C. Act as the body’s natural pain relievers and to enhance feelings of pleasure.  
• D. Regulate our fight-or-flight response to an external stressor.  

Answer: (C)

Verified step by step guidance

1. Identify the term 'endorphins' and understand that it refers to neurotransmitters produced by the central nervous system and the pituitary gland.
2. Recall the primary functions of endorphins, which include their role in modulating pain and emotional responses.
3. Differentiate endorphins from other physiological regulators such as those controlling circadian rhythms, muscle contractions, or the fight-or-flight response.
4. Recognize that endorphins act as natural painkillers by binding to opioid receptors in the brain, reducing the perception of pain.
5. Understand that endorphins also contribute to feelings of pleasure or euphoria, which is why they are often called the body's natural 'feel-good' chemicals.