The first thing you notice is the cooling. The sandstone has been absorbing sun all day, and the moment the last edge of light leaves the rock, it begins to release that heat into the air. You can hear it sometimes — a faint, slow ticking, as cliffs that have been at 50 °C find their way back to 15.
Stars come quickly. There is almost no light pollution in the protected area. Half an hour after sunset, the first major constellations are out. An hour after, the Milky Way is visible as a milky river of light across the whole sky.
And the desert wakes up. Most Wadi Rum mammals — fox, hedgehog, jerboa, gerbil, dormouse — are nocturnal. They emerge from cracks in the rock and from sandy burrows. You will probably not see them, but you may hear them, especially the dry tap of a fox crossing a dune.
Falcons and eagles roost during the night. Reptiles and scorpions slow down. The desert is at its safest when it is dark, oddly enough; the heat is what kills here, and the heat has gone.
Lie flat on a mattress, head on a folded jacket, and look up. Three thousand years ago, a Bedouin lay in the same position and saw the same Orion rising in the east. The sky is the one thing that has not changed.