All mammals, almost all monotremes, and many birds demonstrate the electrophysiological state of REM sleep. Most humans, even those with extensive neurological damage, have episodes of REM sleep which persist even in very old age.

The dreams of all electrophysiologically described sleep states differ phenomenologically from each other. REM sleep dreams differ from other dreams. The best evidence for this difference is in the number of words and length of report—REM sleep dreams are longer than other dreams [32]. In part, at least, due to their length, REM sleep dreams are more likely to be organized into full narrative structures than are other dreams [4, 9]. Because of their length and narrative structure, REM sleep dreams are often the classic big dreams of psychoanalysis, creative discovery, religious, and ecstatic insight. The narrative structure associated with REM sleep dreams most resemble the narrative genre of personal experiences that we call “soap operas” [33].

Nightmares. Nightmares are the most common of all the parasomnias. They are more closely tied to REM sleep than are other dreams. Except in individuals with Post-traumatic stress disorder (PTSD), nightmares occur almost exclusively during REM sleep. Five percent of the general population report nightmares to be a problem, and among insomniacs, nightmares are reported at even higher frequencies [34]. For those who suffer from nightmares, it is the associated distress, rather than the frequency of occurrence that is most closely tied to waking dysfunction. When an individual experiences significant psychological or physical trauma, recurrent nightmares can mark the failure of CNS systems involved in emotional processing [35]. Nightmares are the most commonly reported symptom of PTSD. Recurrent nightmares that disturb sleep in individuals without a history of trauma denote the presence of nightmare disorder [36].

REM Sleep Parasomnias. The other REM sleep parasomnias that frequently include dream content include sleep paralysis; that often includes dreams with negative and frightening content, developed in great detail, and associated with distress that extends into awakening. The dreams of REM sleep behavior disorder (RBD) can be physically aggressive interactions in which the dreamer is attacked by unfamiliar people or animals. Individuals with RBD sometimes exhibit complex and violent behaviors associated with their dream mentation. Injuries to the sleeper or bed-partner are the most common symptom that leads them to seek medical attention. On video-polysomnographic monitoring, most of these individuals demonstrate increased muscle activity during REM sleep, a state that is normally characterized by motor atonia. This motor block that keeps us from moving during dreaming—fails, and individuals with RBD apparently act out their dreams. The parasomnias of RBD sometimes occur outside of REM sleep [37]. Dreaming bizarreness (both scales) and thought processing characteristics as based on work by Wolman and Kozmova for REM sleep dreams are summarized in Box 14.1 [38].

Adapted and updated from Pagel and Helfter [16].

Some clinical trials did not query as to dream-associated drug side effects. This is particularly true for older medications >20 years.

Despite high dream recall frequency (>80 %), the cognitive associations of sleep onset (Stage 1) have received only limited study. Most of what we know of sleep-onset dreaming comes from the study of hypnagogic parasomnias. Hypnagogic hallucinations commonly occur in otherwise normal people at the onset of sleep. These are true hallucinations, most often auditory but frequently visual, and seemingly real to the dreamer. These experiences can be quite bizarre and frightening, ranging from the sounds of a dog barking, a baby crying, or an alarm ringing that wakes the dreamer, to the extreme experience of suffocation at the hands of a succubus. Artists and writers have sometimes induced hypnagogic dreaming and used the resultant dream experiences in their work. Both Salvador Dali and John Keats, known to have used this technique, would attempt to fall asleep while sitting in a chair and holding a coin between thumb and index finger. When they fell asleep, their hands would relax, and the coin would fall into a dish set beside the chair and startling them to awake. There are those who insist that sleep-onset dreams are less bizarre than REM sleep dreams [23]. The surrealistic images that Salvador Dali derived using this technique argue otherwise. Sleep paralysis, most often associated with REM sleep and commonly present in patients with narcolepsy, can also occur at sleep onset. More than half the Americans queried report having experienced either sleep paralysis or hypnagogic hallucinations [39].

Sleep-onset dreams differ from the dreams of other stages of sleep. They are short in duration. They include intense visual imagery and only limited content or story. They can be reality based (particularly in individuals with PTSD) and are sometimes associated with intense distress and anxiety. Each occurs at the initiation of perceptual dissociation that marks the onset of sleep, forming a snapshot of non-perceptual visual consciousness present at sleep onset. These sleep-onset dreams convey an apparent perceptual consciousness but without perception and with limited associated content and memories, without control, yet with intense emotion, visual intensity, and detailed recall. Recent brain scanning studies indicate that compared to other sleep/dream states, large areas of the visual cortex continue to maintain activity despite the eyes-closed perceptual isolation of the state [40]. Sleep-onset dreams include apparently “bizarre” hallucinations, extreme emotional distress, and intense disassociation from reality, yet based on the Hobson scale, sleep-onset dreams due to their short and limited storyline scale are consistently rated as less bizarre than REM sleep dreams. Sleep-onset dream scores are completely different when based on alternative bizarreness scale scores developed by Hunt [27]. Formal characteristics, comparative bizarreness per scale, and characteristic dream state associated thought processes are included in Box 14.2.