[Mithalwen]

Could be either or both ....... both Rivendell and Lorien are places where restful sleep are found and while they ARE perhaps the safest places in Middle Earth due to the power of Galadriel and Elrond to protect them - there may be more to it than that ... I think when they arrive in Lorien Galadriel tells them that they will sleep in peace despite the grief at the loss of Gandalf .... and it maybe that the elves have some power beyond providing a safe environment. I read somewhere in HoME maybe ...... that the finest elvish singers could make their listener "see" what they were singing about ...... but since Elvish sleep and dreams are apparently rather different to mortals ..... it is perhap a hard matter to judge on..