Final answer:
The internal biological clock in the human brain is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, which regulates circadian rhythms. The pineal gland is involved in these rhythms by releasing melatonin, but the SCN is the primary structure that functions as the clock.
Step-by-step explanation:
The part of our brain that is considered the internal biological clock is found in the hypothalamus. Inside the hypothalamus, there is a specific area known as the suprachiasmatic nucleus (SCN), which functions as the main regulator of our circadian rhythms. The SCN receives input from light-sensitive neurons in the retina, allowing it to sync our internal clock with the external environment. While the pineal gland also plays a role by releasing melatonin in response to darkness, which affects our sleep-wake cycles, it is not the structure that acts as the biological clock itself.