Shift work disorder is considered a circadian rhythm disorder because it involves a misalignment between the body and the circadian rhythm that regulates the sleep-wake cycle. The circadian rhythm, influenced by natural light and darkness, dictates the release of hormones such as cortisol to promote alertness during the day and melatonin to induce sleepiness at night. Disruption of this rhythm due to shift work can lead to difficulties in sleeping and maintaining adequate sleep duration.