**Core Concept**
Bright light therapy, also known as phototherapy, is a non-invasive treatment that involves exposure to a specific intensity and duration of bright light, typically in the blue spectrum. This therapy is used to regulate the body's circadian rhythms, which are controlled by an internal clock that responds to light and darkness.

**Why the Correct Answer is Right**
The mechanism behind bright light therapy is based on the suprachiasmatic nucleus (SCN), the part of the brain that regulates the body's circadian rhythms. Exposure to bright light in the morning helps to suppress the production of melatonin, the hormone responsible for inducing sleep, and increases the production of cortisol, the hormone responsible for wakefulness. This helps to synchronize the body's internal clock with the external environment, leading to improved sleep quality, mood, and overall well-being.

**Why Each Wrong Option is Incorrect**
**Option A:** Bright light therapy is not used for treating hypertension. While light therapy can have various effects on the body, its primary use is not for managing blood pressure.

**Option B:** Bright light therapy is not used for treating diabetes mellitus. While light therapy can affect glucose metabolism, its primary use is not for managing diabetes.

**Option C:** Bright light therapy is not used for treating depression through the release of endorphins. While light therapy can have a positive effect on mood, its primary mechanism is through the regulation of the body's circadian rhythms, not through the release of endorphins.

**Clinical Pearl / High-Yield Fact**
One of the key factors to consider when using bright light therapy is the timing and duration of exposure. Typically, patients are exposed to bright light in the morning, within the first hour of waking, for a duration of 15-30 minutes. This helps to regulate the body's internal clock and improve sleep quality.

**Correct Answer: C. Seasonal affective disorder (SAD) and non-seasonal depression.**