I think it's just something Night Furies can do. Once in a while, he also sleeps upside down, so maybe it's a similar trait to bats? [Toothless' dark coloring could indicate that his species is comfortable underground. It certainly seems to be true, but there's still so much that Hiccup doesn't know.

Toothless purrs at Christian when he feels a hand on his wing, an animal-like attempt to comfort, maybe, and Hiccup flashes a smile at his dragon. If nothing else, Christian can take solace in the fact that Toothless and Hiccup are so bonded that the dragon will do anything to protect his rider, and his rider will protect a teammate.] What do you think, bud?

[He fires off another ring of purple light that courses down the tunnel, and Toothless' ears perk up, looking to the humans and then down the tunnel. His stance isn't defensive, so it probably isn't something alive.] I think there might be something up ahead?