Have you injured your back? I ask because symptoms often worsen with immobility (as at night) and ease up with some movement when there is tissue damage.
For short-term comfort I would take some extra magnesium after dinner and apply some Arnica Gel at approximately 7pm and then again when you go to bed.
If you finish the working day and put your feet up early evening your back will have spent several hours in one position before bed, hence the stiffness.
If the pain is affecting your sleep then please consider using our Dormeasan sleep remedy as Valerian was traditionally used for both sleep and tension and could really help you.