Before you head into bed tonight think about how you are going to sleep. If you have no low back, hip, or sacral pain then you may not take this advice seriously. However if you have low back pain, hip pain, are pregnant, or experience sciatic like symptoms you may want to go to bed tonight and sleep on your side with a pillow between your legs. This will help keep the hips from twisting and will maintain your spine in a more neutral position. If you do have symptoms of sciatica then it would be best for you TO NOT sleep on the side of your symptoms even with a pillow between your legs.
As comfortable as this position is, It is also good advice to try to prevent getting into a full fetal position when sleeping. This keeps your spine in a complete flexed position causing strain on the discs and ligaments in your back.
As hinted at previously, whether you have symptoms or not sleeping on your side with a pillow between your legs is best to maintain good alignment throughout your hips and spine and is a good way to prevent/ease morning low back pain.