Elves cannot sleep. They are gifted with being sparred the burden of reliving their past msitakes and fears in the form of dreams.
Information only privy to the party:
A potion can be concocted to allow elves to dream. One was found in the Old Lady of the Forest’s house, the other method is accepting the beverage of the Spider Queen as stated by an eladrin prisoner before his execution.
Once allowed to dream, an elf may choose to dream or not. They are no longer immune to the sleep spell.
Eladrin disappeared :
The eladrin mysteriously vanished from the Prime Material plane from one day to the next nearly some 100 years ago. This was called the “Great Exodus”. They are assumed to have travelled to the Feywild but the few remaining refuse to share this information.