But aren’t sleep hallucinogens just dreams? Yes. Anything we might call a hallucinogens during waking we would call a dream during sleep and dreams can include all manner of weird perceptions: narrative, visual, auditory, and tactile.
When you hear people speak of sleep hallucinations they generally aren’t talking about mental phenomena that happen during sleep, but in fact during waking. Or more precisely, during the waking time very close to sleep or in the transition between sleeping and waking.
You may have friends who describe the weird experience of taking Nyquil or similar nighttime medicines and how they almost think they are seeing zombies while lying in bed. With these experiences, the subject is usually level-headed enough to know the zombies aren’t real, yet they know they are also experiencing an altered reality in the spaces and times between sleep and waking. True hypnagogic hallucinations are different and happen without drugs.
Hypnagogic hallucinations can happen at any time during the night, not just when the person is falling asleep. The medical field has known about this for hundreds of years. It has long been regarded as a curiousity, and to some extent still is.
Some make the distinction between hypnagogic hallucinations (which take place in the transition from wakefulness to sleep) and hypnopompic hallucinations (in the transistion between sleep and waking.) People are sometimes reluctant to admin they have these hallucinogens for fear of being identified as schizophrenic. Elderly people who are going deaf sometimes experience auditory hallucinations that are musical in nature. Subjects sometimes describe these musical experiences as elaborate.
Brain injury victims (including stroke victims) sometimes have trouble keeping their two brain halves together. Called hemispacial neglect this causes perceptions much different from what healthy people experience. It has been found that during the period of dormiveglia a temporary hemispacial neglect can occur as one half of the brain is asleep and the other is awake. This may explain why people often experience hallucinagens (visual and auditory) coming from one direction rather than the other. The direction is usually from the right, as the left side fades from perception faster. The hallucinagens experiences during sleep paralysis – hypnagogia – can have this directional component.
Hallucinogenic drugs also affect sleep. These substances affect serotonin levels in the brain and work on serotonergic neurons http://serendip.brynmawr.edu/exchange/node/1880 Serotonin is one of the key neurotransmitters and is tied up with drowsiness and sleep regulation in ways not totally understood. The psychological affects of hallucigens can include “bad trips” that convince the subject of something awful and may make sleep scary. Some with bad LSD experiences report they fear they will die if they go to sleep.