**Core Concept:** The sleep-wake cycle in humans is regulated by two main processes: the circadian rhythm and the homeostatic process. The circadian rhythm is influenced by the suprachiasmatic nucleus (SCN) in the hypothalamus, which is entrained by external factors such as light and dark. The homeostatic process is determined by the accumulation of sleep pressure as an individual stays awake for longer periods.

**Why the Correct Answer is Right:** When external causes like light and dark are removed (e.g., in a sleep laboratory or during sleep under constant routine conditions), the circadian rhythm is disrupted, leading to a decrease in alertness and cognitive performance. This disruption results from the inability of the SCN to be entrained by external factors, causing a delay or advance in the sleep-wake cycle.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because removing the sleep pressure (homeostatic process) alone does not fully explain the sleep-wake cycle. The circadian rhythm remains a crucial factor in determining sleep-wake cycle regulation.

B. This option is incorrect because removing only the homeostatic process (sleep pressure) does not account for the circadian rhythm and its influence on the sleep-wake cycle. The circadian rhythm remains a significant factor in sleep-wake cycle regulation.

C. This option is incorrect because removing both external factors (light and dark) and internal factors (homeostatic sleep pressure) would result in a complete inability to sleep or remain awake, which is not the case in humans and is not relevant to the question.

D. This option is incorrect because removing only the external factors (light and dark) does not account for the homeostatic process and its influence on sleep pressure. The circadian rhythm remains a crucial factor in determining the sleep-wake cycle.

**Clinical Pearl:** Maintaining a consistent sleep schedule and exposure to natural light and dark cues can help regulate the circadian rhythm and improve sleep quality. Inadequate sleep or disrupted sleep-wake cycles can lead to sleep disorders and affect overall health and well-being.