Sleep paralysis (SP) is characterized as an inability to move while feeling fully awake and aware of your surroundings [1]. Though most muscles cannot be moved, reports often involve the ability to move the eyes. The eyes are usually closed during SP, but they can be opened [2]. SP happens, in most cases, upon waking up or when falling asleep. The experiencer is conscious and can afterwards describe the full event in detail. The inability to move is most often accompanied by fear and, often, frightful hallucinations. These can include figures, sounds, pressure, or the sense of a presence. Further, 75% of all SP episodes are estimated to contain these hallucinations [3]. The hallucinations are often thought to be pictures of dreams, which intrude into the waking perception. Recently, it was argued that SP does have some sort of “aura”. It was found that auditory, tactile, and visual perceptions may prelude SP [4].

SP is connected to rapid eye movement (REM) sleep. REM sleep is one of the stages of sleep, together with non-REM (NREM) sleep, which further consists of three separate sleep stages (N1, N2, and N3). REM sleep is characterized by fast, jerky eye movements and high dream recall when awakened from this sleep stage. Further, it is characterized by muscle atonia, which is thought to occur so that the experienced dreams are not acted out. As the brain is still sending signals to move the body according to the dream content, muscle atonia while in REM sleep is important, so as not to harm oneself or others while sleeping. This is theorized to be the reason that one cannot move during SP [1].

Cheyne and Girard [5] classified the occurring hallucinations into three categories: intruder, incubus, and vestibular-motor (V-M).

OBEs are often treated as an entirely different phenomenon than SP and also have similarities to lucid dreaming (LD). Like in LD, in OBE, the experiencer can often move around freely, according to their own will. Further, V-M hallucinations are often accompanied by positive feelings, including bliss, which is often stated as a common feeling in LD and in OBEs. In this, the V-M hallucinations are different from intruder and incubus hallucinations, which are usually accompanied by fear. Intruder and incubus hallucinations can be isolated but usually co-occur.

Different studies found different percentages on how much of the SP is actually connected to fear. The percentages range from 80 to 90% [1]. In the same range, different studies found similar percentages on how many episodes of SP were pleasant and included positive feelings, such as bliss. About 16% to 17% of participants seem to experience pleasant SP [6,7].

Occurrences of SP are well known throughout history and in different cultures. However, they are not uniformly called SP, but vary through different cultures, who found different names for the often-mythical creatures they made responsible for the episode. These creatures are most likely seen as accompanying hallucinated figures during the episodes. For example, in Italy, the Pandafeche shows up during episodes where one cannot move while lying in bed and feeling crushed; in Newfoundland, it is the “Old Hag”; the kanashibari in Japan; the Jinn in Egypt; in Nigerian culture, a female demon is blamed; and Canadian Eskimos say that spells of shamans cause SP [8,9]. Even the origin of the name nightmare is thought to be rooted in the word mare, which is some sort of goblin, which afflicts one in their sleep with the feeling of suffocation [3]. Further, alien abductions and UFO encounters can occur in SP, and it has been argued that at least a proportion of alien and UFO encounters can be explained by SP [10,11].

The physiology of SP resembles the physiological correlates of REM sleep. However, heart rate is most often elevated during SP [12], which is probably due to fear, as this can also be seen in nightmares [13]. Similarly, SP is also associated with nightmares [14]. However, it has also been argued that SP is a “mixed” state between REM sleep and wakefulness [2]. During SP episodes, EEG activity has been found with abundant alpha activity, which is normally associated with waking, mixed with more typical REM EEG activity [15,16].

LDs are dream states in which the dreamer is aware of being in a dream, and with this realization, they can choose to alter the content of the dream. However, it has been stated that only a subset of LDs involve control. Often in dreams, the dreamer wakes up upon realization that one is dreaming. In one study, only 37% of those who said to be lucid dreamers reported that they could manipulate their dreams [17]. It is estimated that about 55% of people have experienced one LD or more in their lifetime, and about 23% experience LDs about once a month or more often [18].

The term “lucid dreaming” itself was coined by Frederik Willems van Eeden [19]; however, the phenomenon was described a long time before that, and LD was used by Tibetan Buddhists, as well as in Sufism and Indian yoga [20].

LDs can also be used in therapeutic settings [21]. They are able to reduce suffering through nightmares, anxiety, and depression [22,23,24]. Further, LDs have found some use in scientific research. It has been found that using lucid dreamers can give prior agreed-on signals, which can be detected by researchers to detect LDs. For this, moving the eyes left, right, left, and right has been used, which can then be detected by a polygraph [25]. However, recently, a new technique has been proposed, which uses chin movements to indicate LDs and can be detected via EEG. This would allow for easier detection of LD, as this uses less equipment and might be easier on the participants [26]. It has also been found that signals can be sent into dreams, which can induce LDs [25]. Although LDs are praised as a way to combat nightmares, and as being full of bliss and positive emotions, Aviram and Soffer-Dudek [27] have shown that deliberate LD induction can lead to an increase in dissociation and schizotypy symptoms, and it has been argued that, in fact, LDs might not be recommendable for everybody [21].

LDs most often occur during REM sleep, although there have been reports of LDs in NREM sleep [28]. It has been proposed that LDs are a “hybrid” state, combining both elements of dreaming and waking [29]. There are various findings about how LDs differ from regular dreaming in neuronal activation [30]. Still, it has recently been argued that LDs are in fact happening in REM sleep and not in a mixed state [31].

Just like SP, LDs have also been argued to be responsible for at least some alien and UFO encounters and to be connected to the supernatural [32,33]. In monotheistic religions, dreams were often seen as being sent from God. One Christian philosopher viewed LDs as a kind of preview of the afterlife. Therefore, in this way, LDs also differ from SP, as they are sent from God, while SP is mostly the work of demons and spirits who want to cause harm [34].

Therefore, both LDs and SP are connected to REM sleep. Further, both are characterized by heightened awareness compared to regular dreaming. Further, both LDs and SP have been described as hybrid states between REM sleep and wakefulness, which are characterized by high cortical activity and mixed neuromodulation. In LD practitioners, it is common to use certain induction techniques to induce an LD. There have been reports that these techniques may also lead to SP [35]. Previous studies also showed that both LDs and SP are connected to creativity [36,37,38,39]. Further, both phenomena are connected to supernatural and extraordinary experiences. However, while LDs are mostly active, in SP, the experiencer can most often not move and cannot take an active role. Further, SP is often connected to fear, while LDs are usually filled with emotions such as bliss.