The semi supine is brilliant for looking after your back and also resting in a more constructive way than lying in bed. It is the best position for maximum rest for your spine. When lying down it is important to lie on a hard surface such as the floor or a table as this will then give your body appropriate support and sensory feedback about where it is in space.
Lie down with a few thin paperbacks, computer manuals etc. Anything is suitable as long as it has a proper spine, so think books rather than magazines or a cushion. Use enough books so the head is level or tilted slightly forward rather than backwards. The head should be placed so the bony part of the skull is in contact with the books rather than the neck.
Build up to 15 to 20 minutes every day which will give enough time for the body to release onto the floor. If you lie down regularly you may occasionally get leg tremors (shaking). This is nothing to worry about, in fact it is a good thing as your body is releasing tension but may feel quite strange if you are not expecting it to happen. If 15 to 20 minutes seems unmanageable start with five minutes. It will still do you good, and you can build up to longer over time.