Why does Clarisse like to stay up all night walking and watch the sunrise?

Clarisse is very different from most of the people in her society, as is the rest of her family. She doesn't like to be joined night and day to technology, even though that is what is considered normal behavior in this dystopia. Instead, she enjoys experiencing nature for itself, in its reality, rather than by watching it on a screen. She tells Montag that it's important to walk in nature so that you can experience it slowly and fully. She says:

"I sometimes think drivers don't know what grass is, or flowers, because they never see them slowly," she said. "If you showed a driver a green blur, Oh yes! he'd say, that's grass!"

Clarisse represents the old-fashioned world of people living close to nature and actually talking to each other, a world that the novel mourns as lost. Clarisse and her family talk and laugh together. Clarisse does old-fashioned things:

Once he [Montag] saw her shaking a walnut tree, once he saw her sitting on the lawn knitting a blue sweater, three or four times he found a bouquet of late flowers on his porch, or a handful of chestnuts in a little sack, or some autumn leaves neatly pinned to a sheet of white paper and thumbtacked to his door.

Because of his encounters with Clarisse, something begins to awaken inside Montag's soul. His buried dissatisfaction with his existence begins to come to life.