In medical science, abnormal circadian cycles fall under the category of circadian rhythm disorders (CRD). These are separated into two groups based on what causes the condition. External changes cause extrinsic CRDs. For example, a person who works the night shift is not getting much daylight, and this will affect her circadian rhythm. Intrinsic CRDs are more serious because the problem is internal. Emotional, hormonal, or genetic complications can cause these disorders.
The conditions linked to CRD include

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