Turtles are cold-blooded, which means they can’t regulate their own body temperature.  To help them stay warm, they LOVE to sit in the sun.  Ever take a walk by a pond and see a pile of turtles on a rock?  This is called basking and it is very important for the turtles’ health.  Not only does this help them stay warm, but the UV rays from the sun help their metabolism process vitamin D.  They need this to grow properly.  If they cannot get the UV light they need, they can develop lots of health issues, like Metabolic Bone disease.  Turtles are best kept outside in the wild, but if you choose to keep one indoors, you’ll need to provide adequate, full-spectrum UVA/UVB light.  Different species  have slightly different UV light needs and, while there is nothing as good as natural sunlight, it is possible to provide adequate light for turtles kept indoors.  As always, do your research before bringing a pet turtle into your home!