In studies in which volunteers spend several days without access to information about day or night, their sleep–wake cycles

• A. became nonexistent.
• B. shortened.
• C. stayed exactly the same.
• D. lengthened.

Answer: (D)

Verified step by step guidance

1. Understand the concept of circadian rhythms, which are physical, mental, and behavioral changes that follow a 24-hour cycle, responding primarily to light and darkness in an organism's environment.
2. Recognize that the absence of external cues like light and darkness can disrupt these circadian rhythms, leading to changes in sleep-wake cycles.
3. Consider the role of the suprachiasmatic nucleus (SCN) in the brain, which acts as a master clock, regulating circadian rhythms based on light information received through the eyes.
4. Acknowledge that in the absence of light cues, the body's internal clock may default to its natural cycle, which is slightly longer than 24 hours, leading to a lengthening of the sleep-wake cycle.
5. Conclude that without external cues, the sleep-wake cycle tends to lengthen, as the body's internal clock operates on its intrinsic rhythm, which is typically longer than the 24-hour day.