While Galton singled out this description for its clarity and detail, Henslow was only one of many who described essentially similar visions when they were in a quiet, darkened room, ready for sleep. These visions varied in vividness, from faint imagery such as Galton himself had to virtual hallucination, though such hallucinations were never mistaken for reality.
Galton did not regard the disposition to hypnagogic visions as pathological; he thought that while a few people might experience them frequently and vividly whenever they went to sleep, most (if not all) people experienced them at least on occasion. It was a normal phenomenon, although special conditions — darkness or closing the eyes, a passive state of mind, the imminence of sleep — were needed to bring it out.
Few other scientists paid much attention to hypnagogic visions until the 1950s, when Peter McKellar and his colleagues started what was to be a decades-long investigation of near-sleep hallucinations, making detailed observations of their content and prevalence in a large population (the student body at the University of Aberdeen) and comparing them with other forms of hallucination, especially those induced by mescaline. In the 1960s, they were able to complement their phenomenological observations with EEG studies as their subjects passed from full wakefulness to a hypnagogic state.
More than half of McKellar’s subjects reported hypnagogic imagery, and auditory hallucinations (of voices, bells, or animal or other noises) were just as common as visual ones. Many of my own correspondents also describe simple auditory hallucinations: dogs barking, telephones ringing, a name being called.
In his book
I seem to hear the telephone ringing just before I am completely awake in the morning. At first, I would go to answer it, but find that it was not ringing. Now I simply lie in bed, and if the sound is not repeated, I know that it is imaginary and don’t get up.
Antonella B. hears music as she is falling asleep. The first time it happened, she wrote, “I heard a really nice classical piece, played by a big orchestra, very complex and unknown.” Usually, no images accompany her music, “just beautiful sounds that fill my brain up.”
Susan F., a librarian, had more elaborate auditory hallucinations, as she wrote in a letter:
For several decades, just as I am drifting off to sleep, I have heard sentences uttered. They are always grammatically correct, usually in English, and usually spoken by a man. (On a few occasions they were spoken by a woman and once in a language I could not understand. I can recognize the differences between the Romance languages, Chinese, Korean, Japanese, Russian, and Polish, but it was none of these.) Sometimes the sentences are commands, such as “Go get me a glass of water,” but at other times they are just statements or questions. During the summer of 1993, I kept a log of what I heard. Here are some of the sentences: “Once he was walking in front of me”; “This is yours, perhaps”; “Do you know what the photo looks like?”; “Mama wants some cookies”; “I smell the unicorn”; “Go get a shampoo.”
What I hear bears no relationship to what I have read, seen, experienced or remembered on that day, previous day, week or year. Frequently when my husband is driving and we are on a long trip, I will nod off in the car. The sentences come very rapidly then. I will nod off for a second, hear a sentence in the twilight of waking, repeat the sentence to my husband, and then nod off again, hear another sentence in the twilight and so on, until I decide to wake up and stay awake.
In
