A scaredy-cat by nature and a swordsman by force, Demon Slayer's Zenitsu Agatsuma struggles with sleeping on the job. Due to his skittish personality, he is portrayed as being so scared that he passes out while fighting his foes. In the Demon Slayer universe, it is not unusual to see overpowered sword techniques and enhanced sense-related abilities; however, Zenitsu is a strange exception. Only when he is asleep can he draw on the training and technique that he has learned from his master, Jigoro Kuwajima, and his near-death experiences throughout the series.
One of the main questions that many fans may have about Zenitsu is his ability to fight while asleep. Multiple times throughout the Demon Slayer franchise, Zenitsu passes out for several seconds or multiple minutes and then enters into a fully-focused mental state where he decimates the enemy easily. Modern-day sleep science may hold the key to finding out how this phenomenon is even possible and what this means for Zenitsu moving forward in the series.
Demon Slayer: Why Did Zenitsu Get a Sparrow Instead of a Crow?
The Trigger for Zenitsu's Terror
It is no exaggeration to say that nearly everything that speaks, moves, or breathes scares Zenitsu. Having enhanced hearing doesn't seem to help, as it only worsens his fears of enemies that are far away. Throughout season one of Demon Slayer, Zenitsu is shown to fear demons, death, and darkness. Every time that he is face to face with a demon, his whole body freezes, his mouth chatters, and his eyes widen. During the nighttime, he shivers and shakes and holds onto Tanjiro or Inosuke for dear life. As a demon slayer whose job is to fight in the dark, his fully-conscious self doesn't seem that fit for his occupation. However, that all changes when he's asleep as he becomes extremely strong.
The Science Behind Sleep Fighting
As crazy as it might sound, several instances of sleep fighting have been recorded by doctors and scientists. According to Inside Edition's interview with Dr. Allen Towfigh, who is a neurologist and the director of New York Oncology and Sleep Medicine:
Fortunately I have not had any patients who committed homicide, but I have certainly had patients who under various circumstances have engaged in violent behavior [in their sleep]. One of my patients, who had a black belt in martial arts, broke down his entire dresser cabinet.
Two sleep disorders that could possibly explain Zenitsu's condition are REM Sleep Behavior Disorder and Narcolepsy. REM Sleep Behavior Disorder is a condition where a person will experience vivid dreams and physically act them out. Unlike normal REM sleep, where the body temporarily paralyzes the muscles so that the person doesn't move while they're asleep and harm themselves, this disorder bypasses this prevention system and is a cause for concern.
One of the potential causes of REM Sleep Behavior Disorder is Narcolepsy which is defined as:
A chronic neurological disorder that affects the brain’s ability to control sleep-wake cycles.
Narcolepsy has many symptoms including excess daytime sleeping, cataplexy, sleep paralysis, hallucinations, fragmented sleep and insomnia, and automatic behaviors. There are also two types of the disorder: Type 1 and Type 2. Type 1 is narcolepsy with cataplexy, whereas Type 2 is narcolepsy without cataplexy.
What is cataplexy, and why is it important to understand Zenitsu's condition?
Cataplexy is the sudden loss of muscle tone while a person is awake leading to weakness and a loss of voluntary muscle control. It is often triggered by sudden, strong emotions such as laughter, fear, anger, stress, or excitement.
Zenitsu could possibly have Type 1 which is narcolepsy with cataplexy, as throughout the series he's shown falling asleep every time he panics or stresses. One technical note to make is that a person experiencing cataplexy is conscious during the process, so Zenitsu might actually experience cataplexy and then REM Sleep Behavior Disorder nearly at the same time. While it's impossible to completely understand Zenitsu's sleep process without proper testing, several scenes in the series show obvious symptoms of REM Sleep Behavior Disorder and Narcoleptic episodes.
The Boar Bares Its Fangs, Zenitsu Sleeps
In Season One, Episode Twelve of Demon Slayer, Zenitsu's Thunder Breathing technique, Thunderclap and Flash, is first introduced to the audience. After being chased by one of the demons living in Tsuzumi Mansion, Zenitsu and Shoichi are trapped in one of the rooms with a demon who attacks with his tongue and crawls on all fours.
Upon hearing a threat from the demon who tells him that he's going to guzzle his brain matter through his ears, Zenitsu's brain is overloaded, and he falls asleep out of fear. However, a scene later, as the demon tries to attack Shoichi, Zenitsu unleashes a swift attack slicing a part of the demon's tongue off. Before the demon can comprehend what happened, Zenitsu shoots through the air and decapitates him with his Nichirin Sword. Once the minute-long fight is over, Zenitsu awakens, and thanks Shoichi for defeating the demon in his stead.
The temporary sleep episodes listed as one of the symptoms of narcolepsy may play a part in this scene. Temporary sleep episodes only last a few seconds at most and the person doesn't remember what they do during the episode. It is made very apparent that Zenitsu forgets that he was the one who defeated the demon. This happens again near the end of the episode, as he can't recall saving Shoichi when they fell from the second story of the mansion.
You Must Master a Single Thing
In Season one, Episode seventeen, Zenitsu fights against one of the sons of the Spider Family, who is a group of demons manipulated by Rui, the Lower Rank Five Demon of the Twelve Moons, operating under Muzan's indirect influence. The older son of the Spider Family takes on a terrifying form, as he has the head of a demon and the body of a spider. One of the spider son's abilities is to turn a human into a spider creating miniature versions of himself.
After a comedic bout between them, the spider son's creations try to attack Zenitsu who is taking refuge in a tree. When they are close to reaching him, he passes out and falls off the tree branch. Although unconscious, he safely lands on the floor of the forest and fights the older son of the Spider Family. After a few failed attempts to unleash his Thunderclap and Flash technique, he attacks with an upgraded version of his ability, Sixfold.
During this scene, Zenitsu sees many flashbacks in his mind. Assuming that these fall under the category of vivid dreams, he may have been going through a REM Sleep Behavior Disorder episode. As aforementioned, the patient with a black belt was able to break his dresser, so it is possible for Zenitsu to have utilized the muscle memory gained through his master's training sessions and to have been able to unlock this new ability.
Demon Slayer: Mugen Train
In Demon Slayer: Mugen Train, Zenitsu experiences slight character growth, as he's able to access his abilities when Nezuko is in danger. As he is deeply in love with Nezuko, he seems to instinctively know when she's in danger. Even though he's many train cars away, he comes to her rescue at a literal lightning speed. Even as he clearly tells her that he will protect her, he's shown to still be asleep due to the Blood Demon Art that he's been afflicted with.
This scene is another case in which Zenitsu is under the effects of REM Sleep Behavior Disorder, as he's still stuck in the dream state caused by Enmu, who is the Lower Rank One of the Twelve Moons. Movements while sleeping, especially when they are violent, are extremely flashy, so it's not that much of a surprise the audience is shocked during this fight sequence. While Zenitsu may flounder like a fish out of water when awake, he is an apt fighter and protector when he has nothing holding him back in his sleep state. It'll be worth it for fans to wait and see how much he improves over the course of the Demon Slayer franchise in the future.
Source(s): Inside Edition, Sleep Foundation, IMDb, & NIH.