snug as a bug in a rug

Snug as a bug in a rug means to be feeling very comfortable and warm because you are currently in bed or under the covers. This is the phrase commonly used for children when you are tucking them in at night, where you put the blankets tightly around then similar to a burrito or a bug in a rug. Children often associate this phrase with tucking them in, but it is also used for adults in certain situations.