The desert can get mighty cold in winter- down to 0 degrees at times, but more likely in the + 5 to +12 degree Celsius range. Contrast this with the sunny day and the usual desert breeze and you’ll be cold without a decent bag.
Even in May, when daytime temperatures were over 40 degrees Celsius I didn't find the night particularly warm. Having a thermarest is nice but strangely it doesn't seem to matter as much as when sleeping on damp earth, maybe because the sand is dry. Any kind of mattress works, or none, though obviously you'll be a bit warmer if raised above the ground.
Get a four season bag - down or artificial - that is guaranteed down to –10 degrees or even if you are a cool bod, down to -15. Manufacturers are usually over
optimistic and do their rating with the mummy hood fully up and drawn around the face. If you fancy sleeping a bit more freely get a warmer bag - but always with a full length zip so you can
regulate it if it’s too hot.
Go for a recognised brand such as Rab, Ajungilak or Black’s.