I recommend doing a lower back stretch before going to bed.

Lie on your back and hold your legs with both hands.
While keeping your back flat against the floor and slowly exhaling, pull your thighs toward your abdomen as far as you can.
Hold this position for 10 seconds while feeling the stretch in your lower back, then return your legs to the starting position.

Please do this twice.

It helps loosen the muscles around your lower back and improves blood circulation.

Another recommended exercise is the lower back twist.
Lie on your back, bend your right knee slightly, and then let your right leg fall over to the left side of your body.
Keep both shoulders flat on the floor and your face looking straight up; while exhaling, bring your right leg as close to the floor on the left side as possible, hold for 10 seconds, and then return to the starting position.

Do the same on the other side.

Twisting your lower back helps increase the flexibility of the muscles in that area.