When you are ready to nap, get comfy, and take six really deep, slow, breaths. If you can, breathe in from the nose, and out through the mouth.
Nice, slow, easy. This triggers the relaxation response which is the process of deescalating the stress response and inducing relaxation through the activation of the parasympathetic nervous system.
How to do it: With one hand on the chest and the other on the belly, take a deep breath in through the nose, ensuring the diaphragm (not the chest) inflates with enough air to create a stretch in the lungs and breathe out through the mouth.