Hypnagogia
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Hypnagogia is the experience of transitioning from wakefulness to sleep. During this state, one usually experiences visual, auditory or tactile hallucinations, or sleep paralysis. Usually, one has lost consciousness before entering this state, but it is also part of some induction techniques (WILD). It mainly happens during stage 1 of NREM sleep, and before the onset of sleep.
Hypnagogic phenomena
Visual
Initially, hypnagogia will manifest itself as moving blobs of color (see phosphene), usually luminescent, green or purple, in the form of lines, speckles, and geometrical shapes. Those patterns become increasingly complex and can be easily controlled on demand by focusing on specific shapes or thinking about them. Eventually, they can form simple images, even becoming three-dimensional.
Auditory
These can be faint to loud sounds, including short, nonsensical fragments of speech, everyday noises, or a voice calling one's name. Exceptionally, longer-lasting auditory hallucinations can also be heard, such as music. Some people report hearing very loud noises, like crashes, gunshots or explosions, which is a sleep disorder known as the exploding head syndrome. If voices are heard, they are usually those of known people, or the sleeper's internal monologue.
Other senses
Olfactory, gustatory or tactile sensations are also possible in this state, and they are fleeting in nature. Sometimes, there is also some degree of synesthesia, in which a real stimulus (such as a sound) may trigger perception of flashes or images. Floating limbs, feeling that one's body is in a different position from what it actually is, or changes in perceived body size are common too. Strong moving sensations or out-of-body experiences are usually related to sleep paralysis, and can happen during hypnagogia.
Tetris effect
This is common in people that usually perform repetitive actions in their everyday life. It is named after the video game, since when people have play it for extensive amounts of time, it usually starts to permeate their thought when not playing it, and even their dreams. [1] This effect can occur with other video games, non-video games, or jobs or sports involving repetitive actions. In relation with hypnagogia, it causes hallucinations of the action, or the feeling to be performing it, as the brain "replays" memories of daily events.