Final answer:
The endogenous aspects of circadian regulation refer to internal biological processes, with hormones like melatonin being a key component. The biological clock, influenced by these hormones, maintains the body's 24-hour cycle. Hence, the correct answer is the hormone-related option.
Step-by-step explanation:
The endogenous aspects of circadian regulation are primarily internal biological processes and components that help manage the 24-hour cycle of our bodies regardless of environmental cues. Hormones, specifically melatonin, which is released in response to darkness, play a significant role in circadian regulation by signaling sleep-wake cycles in the body. The biological clock, such as the suprachiasmatic nucleus (SCN) located in the hypothalamus, is an internal timekeeper that sends signals to the body to trigger changes in behavior and bodily processes that have a periodicity of about 24 hours. This internal clock aligns with the external environment, and while light exposure can influence this clock, it is not an endogenous factor but rather an exogenous one.
The correct answer to the question 'What are the endogenous aspects of circadian regulation?' is 3) Hormones because they are produced internally and play a crucial role in maintaining our circadian rhythm.