An interesting thread, Fanny. It reminds me of an article I saved years ago (Telegraph, 14 May 2007, Doctor's Diary, James le Fanu) It might interest fellow sufferers.
Those intense and dreadful feelings of nostalgia, bereavement and sadness while dropping off to sleep, as experienced by Mrs BE and several of her Cambrian friends, are widely reported by others. They represent a form of hypnogogic hallucination caused by changes in brain activity before sleep takes over.
These sensations may include the feeling of the "presence" of some ghostly being, auditory hallucinations such as hearing footsteps or vivid visual images of friends, relatives or bizarre and frightening creatures. They tend to be repetitive and episodic, occurring several times in a week, followed by months without them.
There would seem to be several variations, with one woman reporting something "more like terror honing in from a long distance". Alternatively, they may occur on waking, accompanied by a profound sense of gloomy pessimism about life in general. This can last for several hours and can be prevented, a reader reports, with a small dose of the anti-depressant Dothiepin.*
* please note this article is 15 years old and GPs may no longer recommend this particular medication