Titans drew their primary source of energy from the sun, as they became inactive when deprived of sunlight.

Why do Titans only move in sunlight?

Titans derive energy from the sun, being deprived of it results in lethargic, even unconcious titans while most are incredibly active after the sun rises.

Why can't Titans walk at night?

Although they appear to be alive, they are weakened and immobile due to lack of sunlight. A Titan mentioned by Rod Reiss had the ability to harden its skin and create the giant Walls, similar to that of the Wall Titans.