Frogs are cold blooded animals that become colder or warmer depending on the temperature outside. When the sun sets at night, their bodies are likely to be cooler because it is less warm out. When the sun is out however, their bodies soak up the heat and make them warmer. Amphibians were the first vertebrates to live on land and like their cold blooded ancestors they depend on their external energy sources such as the sun to maintain a stable body temperature.
Reptiles on the other hand are ectotherms and obtain their heat from outside sources such as the sun and regulate their temperature through behaviors such as basking or seeking shade. Their body temperatures are usually about the same as their ambient surrounding’s temperature.