In the wild, mice build nests in their burrows for sleeping and rearing young. Pet mice do the same thing because the instinct to nest is powerful in these small animals.
You’ll know your mouse is building a nest when you see it carrying bedding to one corner of the cage and stashing it there. Eventually, it will burrow itself into the bedding and go to sleep. A nest helps your mouse feel more secure, and helps it stay warm in cool weather.
To help your mouse build a nest, provide it with the material it needs to get the job done. Paper-based bedding is best for this, and your mouse will appreciate the small pieces it can carry to its special corner.
Some mice like to add hay to their nest, as well as shredded paper towels and tissues. You may even want to provide your mouse with a small nest box so it can build a nest inside it.