Is it necessary to change one's underwear every time dampness or white spots are found after sleeping, both at night and during the day, given the difficulty of repeating this?

For the prayer to be valid, purity from impurity is a prerequisite. The moisture or stains in the underwear are either maniy (seminal fluid) or madhiy (pre-seminal fluid). Maniy is pure and necessitates ghusl (ritual bath), while madhiy is impure and necessitates washing the male organ and the affected areas, or changing the clothes for prayer. If a person is unsure whether it is maniy or madhiy, they may choose either. If the stain is impure, only washing it is required, not changing the clothes. Complete purity is not a condition outside of prayer and tawaf (circumambulation of the Kaaba), though it is recommended.