What can cause an increase in body temperature?

An increase in body temperature can be caused by dehydration because when the body lacks sufficient fluids, it can lead to an elevation in temperature. Dehydration can interfere with the body's ability to regulate heat effectively due to reduced sweat production and impaired circulation. This inability to maintain normal body functions can result in hyperthermia or heat-related illnesses.

On the other hand, excessive sleep, cold weather, and reduced physical activity are typically associated with lower metabolic rates and a cooler body temperature. Excessive sleep may not provide the stimulation necessary to raise body temperature significantly. Cold weather often leads to the body conserving heat to keep warm, which typically does not elevate body temperatures but rather helps in maintaining them. Reduced physical activity generally decreases metabolic demands, leading to lower body temperature. Thus, dehydration stands out as the correct factor that can lead to an increase in body temperature.