Please explain why is a post operative patient under sub arachnoid block anesthesia positioned flat on bed for 8 hours?

Spinal headache: The reason why patients are kept in the lying down position after subarachnoid block or spinal anesthesia is to decrease the chance of developing a post dural puncture headache or commonly known as a spinal headache. There is a hole made from the introduction of the spinal needle through the dura and into the subarachnoid space to deposit the medication there to accomplish a spinal block.