A washed window returns light that had been disappearing slowly enough to escape complaint.

Decline that happens slowly enough never triggers a complaint. The glass dimmed over two years at a rate no one could perceive in a day, so the room simply became darker and everyone adjusted their idea of normal downwards. Only the cleaning reveals what had been lost, which is why gradual losses are more dangerous than sudden ones.

Ask what in your life has been dimming at a pace too slow to notice: a friendship, a routine, a skill, the light in a room. Then do the equivalent of washing the glass once and see what comes back. The comparison is the only reliable measurement available.