USABO Practice Exam 2026 – Complete Prep Guide

The mouse's circadian cycle shortens in a constant dark environment primarily because the circadian clock relies on environmental cues, also known as zeitgebers, to maintain its proper timing. In a natural setting, light is the most significant cue that helps synchronize the circadian rhythms with the day-night cycle. When these cues are absent, as in constant darkness, the internal clock can drift and become misaligned with the natural 24-hour cycle.

As the circadian clock continues to operate in a constant dark environment without external signals, it tends to run shorter than its typical 24-hour duration. This drift occurs because the intrinsic rhythm of the mouse's biological processes may not be perfectly aligned with the clock. The resetting of this rhythm usually depends on exposure to light, which helps the organism keep its biological functions synchronized with the cycle of day and night.

Other options may seem plausible but do not capture the core reason for the change in the circadian cycle. For instance, the notion of becoming nocturnal typically refers to behavioral changes in response to environmental light, rather than an intrinsic change in circadian rhythm. The concept of experiencing a loss of biological activity does not address the fundamental mechanism of circadian clock function, and adaptation to new environmental conditions might imply