The answer ranges from a bad idea to truly demonic, depending on what form of altered state of consciousness is implied. This question is interesting and one might not make the connection between hypnosis and faith healing movements but it does have some overlap into certain fringe movements within Christianity, such as the faith healers like Oral Roberts, T.L. Osborn and William Branham. My Uncle actually leads the Branham-ites and the research (even though secular) that I dug up, actually helped me to understand better the occult and quackery side of it.
Dr. James Braid, the Scottish surgeon coined the word "hypnotism," after witnessed demonstrations by a French mesmerist in 1841. Therefore, the modern term is too recent to gather good references from reformed heavy hitters. Yet this need not discourage us, the reformed position can easily be extracted by simply breaking up hypnotism into its historical components and comparing its methods to reformed doctrines.
The history of hypnosis finds its roots in earlier theories developed by Mesmer. He in turn finds his roots in ancient medical philosophy like traditional Chinese medicine and even shamanism. The idea had to do with a belief in animal magnetism and ways of manipulating it to heal people. One had to be brought into a 'state' to experience the 'strange' effects of the magnetism of the planets linked to our bodies, like the ying and yang.
The subject approaches the circumference of reformed theology in terms of Christian 'religious experience'. For example being slain in the Spirit could be basically considered the same state. ( I say may, because I do not discount the possibility of feinting when overcome by a conviction of sin, r wonderful view of God's grace, if a person is feeling a little weak or in a room without a lot of oxygen. )
In some sectors of Christianity there is an overlap between states of consciousness and religious experience, especially among the cults and extreme faith healers that used similar techniques of mesmerists and hypnotizers to bring about a 'frenzied experience' in the hopes of curing psychosomatic illnesses. Worse, when brining in a layer of anti Christian doctrine, in addition to the sleep induced manipulations, or spiritsm, it veers into the occult which aggressively crosses the boundary line of Christian ethics.
To work from a reformed position of the 'ecstatic' or the state of being in a prophetic 'trance' which historical mesmerists would have discounted but explained as religious phenomena that verified their theories, reformed views establish a position that is 'directly opposed' the techniques used in mesmerism that eventually led to hypnosis. We can see this in how John Owen explains the experience of the Holy spirit to be an 'orderly, voluntary creative work' that does not play 'tricks with our minds', or otherwise 'manipulate us' towards a uncontrolled 'ecstasies' or state of consciousness:
“The work of the Holy Spirit in regeneration does not consist in enthusiastical raptures, ecstasies, voices, or any thing of the like kind. Such things may have been pretended to by some weak and deluded persons: but the countenancing of such imaginations, or teaching men to expect them, or esteeming them as conversion to God, while holiness was neglected, is a calumny and false accusation, as our writings and preachings fully testify.”
“Therefore as to this negative principle we observe that the Holy Spirit usually exerts his power in the use of means, and that he works on men agreeably to their natures. He does not come upon them with involuntary raptures, using their mental powers as the evil spirit wrests the bodies of possessed persons. His whole work is rationally to be accounted for, by those who believe the Scriptures and have received the Spirit of truth whom the world cannot receive.”
“Indeed, the efficiency of the Spirit in quickening our souls (which the ancients always termed his ‘inspiration of grace’) is no otherwise to be comprehended than any other act of creating power, for as we hear the wind, but know not from where it came or where it goes, so is everyone that is born of the Spirit (John 3:8). But this is certain, that he works nothing but what is determined and declared in the written word, and that he puts no force on the faculties of our souls, but works in them and by them suitably to their nature.” (A Discourse Concerning the Holy Spirit, By John Owen)
This opposes most of the history of hypnotism including mesmerism and most of the popular forms of hypnotism used in entertainment. To elaborate a little, interesting and concise details of the history of Mesmerism and early science of hypnosis can be found here at this thesis paper on Mesmerism. (Secular source: B. Brilliant)
For example regarding rituals common to Mesmerism and early hypnosis:
Hypnotism and mesmerism share a certain dependence on ritual as a means of concentrating the attention of the subject and impressing upon him that something important and effective is taking place. Hypnotic induction procedures may make use of candles, pendulums, crystal balls, and other objects of focus; hand passes in front of the subject's eyes are often employed; and the verbal portion of the method usually involves leading the subject on a series of steps into deeper relaxation and concentration through directed imagination. For example, the subject may be asked to picture a carpeted stairway that he will descend into a warm, comfortable room, or to imagine himself/herself walking off into a flowery meadow. The subject is led to believe that she/he is, in effect, going somewhere else, into a special state where the rules of everyday life may not apply, so that extraordinary things may happen--even something as extraordinary as giving up smoking. (Secular source: B. Brilliant)
First hand subjects of Mesmer indicate how powerful unstable people are when getting together with high expectation and who open themselves to an ‘experience’:
Some patients remain calm and experience nothing; others cough, spit, feel slight pain, a local or general heat, and fall into sweats; others are agitated and tormented by convulsions. These convulsions are remarkable for their number, duration, and force, and have been known to persist for more than three hours. They are characterized by involuntary jerking movements in all the limbs, and the whole body, by contraction of the throat, by twitchings of the hypochondriac and epigastria regions, by dimness of vision and rolling of the eyes, by piercing cries, tears, hiccough, and immoderate laughter. They are preceded or followed by a state of languor or dreaminess, by a species of depression, and even by stupor. (Secular source: B. Brilliant)
This covers the subject from a charlatan and quackery standpoint, and in deed this is the origin of the science, however their also is the demonic element. Extreme religious groups do more or less the same thing as a ‘road show’ entertainment kind of false healing program and when this is accompanied by teachings that run in direct opposition to the gospel, reformed theology can’t be view this as a demonically enhanced version of simple quackery, although both may be primary interested in making money under the abuse of their clients. For example Mesmersim has lead to Swedenborgianism which is a Christian sect that denies almost every traditional Christian belief as well as supports abelief of out of body experiences with spirits, etc. Also in the article I have linked to we see the background of ‘Christian Science’:
Mary Baker Eddy, the founder of Christian Science, vehemently denounced mesmerism and hypnotism, in the apparent belief that they are totally grounded in the physical realm and unrelated to Spirit or God. But since she studied under the mesmerist Quimby, she must have been aware that her doctrine of the mind's influence on health could almost have come directly from Mesmer's writings. It seems that she was, then, simply maligning a rival in the field of healing. (Secular source: B. Brilliant)
The occultist history of the science is also conveniently found in the article:
The roots of hypnosis are lost in the abyss of the ages. But the human race has always known and explored altered states of consciousness. The peoples of the ancient world knew many of the keys that open the gates to a different reality, such as fasting, meditation, sleep deprivation--and even dance, as we see in the ecstatic states achieved by the whirling dervishes of Turkey; Haitian and African voodoo dancers; and American Indians in their dance ceremonies. Yogis, fakirs, Persian magi, and Christian mystics such as St. Anthony of Padua and St. Theresa of Avila have all made use of trance states.
These mental transformations could also be chemically induced. The legendary Pythia, priestess of the Delphic Oracle in Greece, inhaled sulfurous fumes from a cleft in the rock within the temple, then spoke her strange prophecies. African witch doctors and other sbamen also prepare themselves by taking a substance appropriate for the ceremony they are to perform. (Secular source: B. Brilliant)
However, there is an element of hypnotism that would be more medical and a kind of simple self-induced sleep with simple attempts at the ‘power of suggestion’. This does not have the same abusive, or occultist implications. In this sense a man could even whisper to his wife in her sleep, ‘You do not want to buy purses and shoes’ and see if it has any effect without necessarily sinning. The only thing I would say is that considering the dark history of the subject, often overlapping with occult practices and being pioneered by people strongly opposed to reformed views, a reformed position would remain suspicious to say the least. More than that since, the heart of man is perceived as being 'desperately wicked' and 'deceitful above all things' the idea of subjecting your own thoughts, conscious or sleep induced, to another sinner, where principles and guard of aware discipline against all temptation is willingly surrendered, seems to oppose the concepts of sanctification through the study of God's word. In other words, prayer and discipline are reformed methods for quitting smoking and hypnosis is just various shades of gray, or even black, which can't be as effective or as reasonably reliable. However, as Medical hypnosis does not have these darker undertones, so it would be slotted more along the lines of a bad idea, rather than the demonic.