Hypnagogic hallucinations are hallucinations which occur at the boundary between sleeping and waking. They can occur when people are falling asleep, or when they are starting to wake up, and they tend to be extremely vivid, feeling like a Technicolor Oz after the black and white Kansas of every day life. Many people experience hypnagogic hallucinations at some point during their lives, but recurrent intense hallucinations can be a sign of an underlying medical condition which may require treatment.

Visual, auditory, tactile, and kinetic sensations can all be experienced during hypnagogic hallucinations, and everyone experiences slightly different forms. Some people, for example, may feel like they are falling, and jerk themselves awake to avoid hitting the ground. Others may hear voices as they are trying to drift off to sleep, or experience a vivid sensation that someone or something is in the room. Sensory experiences such as feeling like one is submerged in a pool of water are also not uncommon.

In some cases, hypnagogic hallucinations can be frightening for the people who experience them. They may include vivid and frightening images, including images which are out of scale, which can make the hallucinations seem even more unsettling; people may see giant spiders on the walls, for example, or feel like they have shrunk down to a tiny size in the bed. The vivid experiences may also be brought to mind over the course of the day, causing inexplicable images or sensations to filter through someone's consciousness at an unexpected moment.

The cause of hypnagogic hallucinations is not fully understood. These hallucinations tend to be more common in people with sleep disorders, especially narcolepsy, but they can also appear as a side effect related to prescription drugs, and drug abusers often experience them as well. Hypnagogic hallucinations tend to be more common in young people, especially children, which may be because their minds are still developing and forming pathways, which can occasionally lead to some crossed wires.

If someone experiences numerous hypnagogic hallucinations, repetitive or not, it is a good idea to see a doctor to check for health problems which could be related to the experiences. If no cause is evident, a psychologist or similar mental health professional might be able to explore the subconscious causes and help the patient deal with the hallucinations. Consulting a psychologist can also yield useful tips for people who are shaken or upset by hypnagogic hallucinations, even if the hallucinations continue to occur.