Hi.  Usually viral fever lasts for 7 days.  Generally viral  fever is continuous,  present throughout the day with minor fluctuations in the temperature, unless paracetamol or other medicine is being given to control the fever.  If the fever is just at night times then it is reasonable to do blood test to rule out other causes of fever. A Pediatrician must be consulted in person before doing any blood test to make sure that it is just a viral fever and nothing else...