how much hard is a dungen with no long rest

In a campaign I am still working on the players will be doing a lot of dungeon's.  Because of the magical nature of some of them the will not be able  to leave the and even long rest only count as short rest.  I am thinking of treating these dungeons like they are 2 level higher then normal.  What do you all think about them.

I may make it so you only have a 25% change of doing a long rest and on a fail the 8hour was just a short rest.  Also I will be letting the players know about this before they make thier PC's.