A drop in body temperature following the body's natural sleep pattern may cause one to feel tired, according to PubMed of the National Institutes of Health. Lack of activity when one is feeling tired also can contribute to feeling chilled, as muscles generate body heat as they move.
The human mind and body follow a natural 24-hour cycle called the circadian rhythm, which includes regulation of body temperature, according to University of California, Los Angeles' Sleep Disorders Center. It is natural for the body to drop in temperature when one is sleeping, says Dr. Craig Heller of Stanford University quoted on WebMD. If a person is tired, especially if she is up later than usual, it may be her body's natural time to sleep. The body naturally follows its normal pattern of reducing body temperature according to that person's circadian rhythm, making her feel cold.
In addition, as one becomes more fatigued, her activity level may decrease, causing less heat to be generated by muscle activity. This is one of the reasons that people shiver when they're cold — their muscles are trying to generate heat to warm up the body, according to Education Portal.
If a person is chronically tired and cold, she should to seek a physician's advice to rule out illnesses whose symptoms include of cold and fatigue.