Few epidemiological studies have explored hallucinations’ prevalence and types in the general population. Ohayon et al.  have emphasized the high prevalence of hypnagogic and/or hypnopompic hallucinations present respectively in 37 and 12.5% of the 4972 subjects. The rate of hypnagogic hallucinations was related to age. Women were more likely to report such hallucinations. The most frequent type of hallucinations was unexpected, that is kinesthesic (feeling of falling in an abyss) but all types of hallucinations could be present. These hallucinations had no relationship to a specific pathology in more than 50% of the cases but were exclusively present at nighttime. In old age, visual hallucinations are the most frequent. In a cohort of old non-demented subjects, 17.4% of them had hallucinations and behavioural symptoms (anxious agitation or/and irritability).
Several neurological pathologies included sleep disorders and hallucinations. Narcolepsy and Parkinson disease (P.D.) will be only considered here. In narcolepsy, with or without cataplexy, hallucinations are now called “secondary” symptom, as they are incidental, reported in 35 to 66% of the patients.
Hypnagogic and hypnopompic hallucinations are most often visual but all types have been described, formed, without whole scenes but terrifying the patient who is often implicated in it and sometimes associated with sleep paralysis, especially during daytime when sleep occurs in REM. Are they pieces of dream? These hallucinations can be differentiated on a phenomenological basis from hallucinations in healthy subjects and from hallucinations in schizophrenia . They have also some differences with hallucinations observed in P.D. .
In P.D. hallucinations affected 25% of the patients in long duration disease. They are mainly visual, or with sensation of a presence (person or animal), coexisting with delusions and anxiety. They are rarely only a side- effect of dopaminergic treatment but are often linked to daytime sleepiness with REM sleep attacks, described in this disease and to cognitive impairment.