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The fallacious stipulation is that we are only conscious when we are awake; the definition of consciousness encompasses the state of wakefulness and it has boiled down to whether or not our eyes are open or shut. That being said, humans spend a third of their lives in a state of repose and the science of consciousness has allowed scientists to discover the existence of brain activities during certain activities that, upon analysis and contemplation, has given birth to the answer as to whether or not we truly are conscious. This is accentuated by the existence of consciousness during the sleep cycle — a duration of time in which humans are asleep — and the flaw in the postulation that humans are not conscious when asleep is that they actually are, just minimally. This has been proven by a plethora of studies conducted via transcranial magnetic stimulation (TMS) and default mode network (DMN) that show the existence of consciousness albeit minimal during the sleep cycle through observing certain activities and mentation (brain activity) within specific sections of the brain. 
A major issue in defining consciousness is the mystification of it that is due to its complexity. It is not something that is empirical or quantifiable, and so the principal problem of consciousness is how to integrate our feeling of subjective experiences with a scientific worldview; despite this, consciousness can still be defined in a physical sense. According to various scientific evidence, consciousness can be attributed to a high degree of activity within the cerebellum, also known as “the little brain”, or the “sensorium”, or seat of feeling (Gore, 1905). The existence of our consciousness cannot be proven to exist without the nervous system (Cavanna, Shah, Eddy, Williams, and Rickards, 2011). Conscious experiences are unique; they are inherently private and personal and they varies from person to person, which is why progression in the science of consciousness has been slow. Consciousness may be dependent on the nervous system, as it is the system that transmits information to the brain, where our mind coexists, but the properties of consciousness cannot be reduced scientifically. Reduction is the process by which objective phenomena is reduced to more fundamental objective phenomena, on which the former supervene (MacLennan, 2003). For example, heat is caused by the kinetic energy of molecules, but it cannot be said that consciousness can be reduced to anything particular brain function/region. However, we can circumvent the subjective definition of consciousness by loosely defining it as the state of being cognizant or aware or sentient of oneself and the environment. It is to be alert and conscious and at the very least, being conscious implies that one has some level of mindfulness.
Our consciousness, however subjective, is still dependent on — but not reduced to —memory, observation, perception, attention, comparison, inference, and imagination (Baars, 2009), all of which is attributed to our senses reacting to the natural energy of the environment. Because humans receive stimuli from all parts of the body, it is only natural that the sensorium becomes excited through external and internal influences that vary in strength (Gore, 1905). Therefore, consciousness has different levels, from the faintest perception to the greatest pain or pleasure, or from a peaceful sleep to a raving madness. However, most scientists remain firm in a positivist and reductionist epistemological approach to the sphere of consciousness (Taylor, 2005). Traditionally, this view demands adherence to the idea that there is no other state than the normal everyday waking one. All phenomena are merely variations of this one single state. When one is asleep, according to this view, there is no consciousness. Consciousness and awareness are identical. There is nothing else. There is no such thing as an unconscious, nor is there any reality to the idea of multiple states of consciousness, let alone higher or lower ones, as these are all thought to be mere projections of the waking state. Franz Anton Mesmer, healer of psychosomatic and hysterical illnesses corroborated this when he wrote, “Man’s sleep is not a negative state, nor is it simply the absence of wakefulness; modifications of this state have taught me that the faculties of a sleeping man not only are not suspended, but that often they continue to function with more perfection than when he is awake.” (Taylor, 2005). He iterates his belief that sleep is not implicative of unconsciousness (“an absence of wakefulness”) but he has learned that it is the polar opposite; we function even better when asleep through the active processes that occur during sleep. This, to Mesmer, is a sign of consciousness during the sleep cycle — the fact that our bodies function even better when asleep.
To determine whether consciousness exists, one must first determine whether any form of brain activity exists during sleep because the brain is essentially the nervous system; any form of activity whatsoever could show that the brain is not asleep but still functional, and that a form of consciousness no matter how minimal exists. Electroencephalogram (EEG) recordings from the cerebral cortex reveal different wave patterns of electrical activity in the brain corresponding to different levels of stimuli (“Stages of Sleep”, n.d.). Brain activity is typically divided into four basic types, according to frequency of waves being formed per second (in hertz, Hz). Beta activity is low amplitude, fast activity occurring at frequencies of 13–30 Hz, dominating our normal waking state of consciousness. Alpha activity, at frequencies of 8–13 Hz, delineates the passive EEG, and is the predominant background activity during the relaxed waking state when the sleeping hormone melatonin is released from the pineal gland and the individual’s eyes are closed. Once the person falls asleep, the alpha waves change into the irregular, drowsy, non rapid eye movement (NREM) stage 1 waves which are the slower theta waves, with a frequency of 4-7 Hz. It is in this first stage of sleep that one may experience hypnogogic sensations, where it may feel as though one is falling and s/he jerks and startles him/herself (Hobson, 2005). As s/he commences to relax more deeply, s/he moves into NREM-2 stage sleep which is characterized by the periodic appearances of short bursts of rapid brain waves known as sleep spindles and the theta waves produced becomes gradual. The person then moves into a deep sleep known as the NREM stages 3 and 4, in which our brain produces even slower delta waves of frequencies less than 4 Hz (Picchioni, Dyun, and Horovitz, 2013). Once the fourth stage of the NREM sleep cycle is completed, the cycle reverses, going back to stage 3 and 2 but not entering NREM-1 stage, rather entering the rapid-eye movement (REM) sleep. 
While it is inherently difficult to prove complete absence of consciousness in state studies, there is no question that deep sleep is much less conscious than full, responsive wakefulness (Cavanna et al., 2011). Humans are therefore still conscious during the sleep cycle, be it “slow wave sleep” (NREM-3 and -4), which is associated with a high proportion of delta activity in the EEG, or REM sleep, during which the EEG resembles that during wakefulness (an amalgamation of alpha and beta waves) (“What are Brainwaves”, n.d.). The latter is known as a “paradoxical sleep” because the motor cortex is very active but the brain stem blocks those impulses, leaving muscles relaxed that one’s body is put in a state of ‘paralysis’ except for our eyes and individuals are more difficult to arouse during REM sleep. Despite the low level of stimuli, REM sleep is associated with the dreaming state and a raised content of consciousness, the exception to the otherwise positive correlation between level and content of consciousness in normal physiological states.
Part of the behavioral definition of sleep is a significant decline in responsiveness to external stimuli. This reduction in responsiveness, or consciousness, during sleep has been attributed to the breakdown of connectivity between brain regions (Achermann, Rusterholz, Dürr, König, and Tarokh, 2016). According to this idea, called the integrated information theory, consciousness arises when information is integrated across brain regions. This study, while it may seem as though it supports the popular belief that consciousness is inexistent during the sleep cycle, in fact proves that there is some form of consciousness during the sleep cycle although significantly reduced. Support for this conjecture comes from experimental studies that have used simultaneous high-density EEG and transcranial magnetic stimulation (TMS) to examine signal propagation during waking and sleep. These studies have found that when compared with waking, in which the TMS-induced signal propagates across brain regions for up to 300 ms, during early NREM sleep, the activity induced by TMS remains localized to the site of stimulation and lasts less than 150 ms (Achermann et al., 2016). As compared to early NREM sleep, the TMS-evoked response during REM sleep, a state often associated with vivid dreams, was more widespread and longer in duration. However, the TMS response during REM sleep was more localized than that observed during waking, suggesting that the response to REM sleep is in between that of waking and NREM sleep. This correlation between brain activity and level of consciousness can also be reflected by the functional integrity default mode network (DMN) which is a network of brain regions in which it was found to reflective of and dynamically modulated by the level of consciousness (Chow et al., 2013). Studies have shown that the DMN remains coupled (connectivity between brain regions is evident) during light sleep and uncoupled during slow-wave sleep (NREM-3 and 4), where the connectivity is significantly attenuated (Horovitz et al., 2009). Therefore, many scientists have postulated that the brain activity that occurs is heavily correlated to level of consciousness during the sleep cycle.
Therefore, researches have evidently shown that brain activities exist during the sleep cycle, whereby it produces alpha, beta, and delta waves. And through studies that have shown a correlation between brain activity and level of consciousness (via TMS and DMN), the brain activity that is existent during the sleep cycle therefore proves that humans are, in reality, conscious even when asleep. The postulate may be that consciousness requires us to be aware of ourselves and our environment, but an active brain could also be indicative of a conscious state in the physical sense. The frequency of the brain waves, no matter how slow, reflects the actual level and state of consciousness that humans are in. Brain activities may be relatively slow during the sleep cycle compared to wakefulness, but it disproves the assumption that humans are not conscious. The extent to which we are conscious when experiencing the sleep cycle, both in REM and NREM stages, is minimal, with the REM stage being the ‘higher’ state of consciousness than that of the NREM stage.

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