What physiological system is disrupted by changes in sunlight, leading to feelings of depression during fall and winter?

• A. Circadian rhythm
• B. Digestive system
• C. Immune system
• D. Nervous system

Answer: (A)

Changes in sunlight significantly affect the circadian rhythm, which is the body's internal clock that regulates sleep-wake cycles and various bodily functions based on the 24-hour cycle of day and night. During the fall and winter months, shorter daylight hours can disrupt this rhythm, leading to an imbalance in hormone levels, including serotonin and melatonin.

The reduced exposure to sunlight can lead to decreased production of serotonin, a neurotransmitter that contributes to feelings of well-being and happiness. Additionally, it can cause an increase in melatonin production, which is associated with sleepiness and can further contribute to feelings of depression. These changes can manifest as Seasonal Affective Disorder (SAD), a form of depression that occurs at a specific time of year when daylight is limited.

In this context, the other physiological systems mentioned, such as the digestive system, immune system, and nervous system, are not primarily impacted by changes in sunlight in the same directly causal way. They may have effects that indirectly relate to overall mood, but it is the circadian rhythm specifically that is directly influenced by sunlight and has a prominent role in mood regulation during seasonal changes.