There are few clouds and little humidity to hold the heat from the daytime. Therefore, the heat rapidly radiates back into space when the sun sets.
A desert cools much more repidly than a swamp because there is little humidity to help hold the heat.
Due to lack of humidity and cloud cover, the desert cools rapidly after sunset.
They are small so in the desert they do give off a lot of heat which keeps them cool.
These deserts have a generally cool climate:Atacama Desert Sechura Desert Monte Desert Patagonian Desert Namib Desert
The Atacama Desert of South America is a cool coastal desert.
Both a desert and a swamp warm up in the daytime, not cool. They both cool at night with the desert cooling the fastest.
Every desert has its own climate statistics so you need to specify a particular desert. There are hot deserts, cold deserts and cool deserts.
The climate of an oasis depends on the climate of the surrounding desert. If the desert is hot, the oasis will be hot. If the desert is cool or cold, so is the oasis. If it is raining or windy in the surrounding desert, it will be the same in the oasis.
The Atacama Desert is a cool, exceedingly dry desert.
The rainy season is short in the desert so plants must grow rapidly so as to complete their life cycle and produce seeds before dry weather sets in again.
So they can not get to hot and so they can wait for there prey undisguised!
The Namib is considered a cool coastal desert.