Waking up with an elevated body temperature of 39 degrees may be caused by inflammatory infections, metabolic reactions, and tumor fever. 1. Inflammatory infections: including viral infections, bacterial infections, fungal infections, mycoplasma infections, etc., all of which may lead to an immune response in the body, thus leading to fever. 2. Metabolic reaction: It may be caused by acute allergy, such as the allergy after vaccination, which often causes fever. It may also be caused by the allergic reaction during infusion, which often causes high fever. 3. Tumor fever: mostly seen in malignant tumors, such as malignant lymphoma, leukemia, etc. If secondary infection, it is also easy to cause high fever. Waking up with a temperature of 39 degrees, there may be caused by other reasons, it is recommended to go to the hospital in a timely manner to identify the cause or targeted treatment.