Internal processes. Modifications of internal processes, in addition to sleep and dreaming, may be used to change an individual's level of consciousness. Hypnosis is a procedure whereby the hypnotist creates in a subject a relaxed, highly suggestible state. In meditation, a person narrows her or his span of attention to produce feelings of relaxation. In biofeedback, a procedure related to meditation, an individual uses techniques to measure bodily processes—for example, heart rate, skin temperature, or brain waves—and learns to modify them, usually in a way that promotes relaxation. External processes. External factors, particularly chemicals such as drugs, may also affect levels of consciousness. Environmental factors such as poor ventilation may influence chemical effects. Chemical substances (drugs) are often used in both medical procedures and daily life primarily to change levels of consciousness. In medical procedures, chemicals called general anesthetics (halothane, nitrous oxide, and others) are used to produce the state of total unconsciousness needed for major surgery. Analgesic drugs, such as morphine and other opiates and aspirin, are used to reduce pain; morphine and the opiates can also cause changes in mood and can become addictive. An addicted person becomes dependent on a drug or drugs and suffers discomfort when it or they are not available. In addition, tolerance to the drug(s) occurs, and larger and larger doses are required to produce the same state of relaxation or euphoria. Tranquilizers (antianxiety drugs) are among the most consistently prescribed mood‐altering (in this case, anxiety‐reducing) drugs. These drugs include such substances as the benzodiazepines, which include the well‐known drug Valium (diazepam). Depressants have a relaxing and calming effect. They include such substances as the barbiturates and ethyl alcohol, widely used as a recreational beverage in beer, wine, and liquor. Both barbiturates and alcohol can be addictive. Stimulants increase activity in the central nervous system. Examples of stimulants are cocaine, amphetamine, nicotine, and caffeine. Cocaine and amphetamine both block the reuptake of neurotransmitters and produce feelings of euphoria; both have been used for medical purposes. Cocaine injections can be used to produce local anesthesia, but crack cocaine (which users generally smoke) and cocaine per se (which is usually injected) are used as recreational/street drugs and can be addictive. Amphetamines are stimulants that produce feelings of alertness; they are prescribed as a diet aid and to treat narcolepsy and Parkinson's disease, but they also are used as street drugs, where they are called speed, uppers, or bennies. They affect the reuptake of the neurotransmitter dopamine and, when taken incorrectly, can create symptoms similar to those sometimes seen in a mental disorder known as schizophrenia. Nicotine is found in cigarettes. The difficulty of breaking the smoking habit is well known. Caffeine is, of course, found in coffee, tea, and a number of soft drinks. Many people depend on their morning caffeine jolt. Hallucinogens produce mood and behavioral changes that mimic psychotic behaviors. The hallucinogen LSD (lysergic acid diethylamide) is a compound that interferes with the reuptake of the neurotransmitter serotonin. Other examples of hallucinogens are PCP (phencyclidine, also known as angel dust) and marijuana (tetrahydrocannabinol). Designer drugs are illicitly manufactured variations of known recreational drugs such as opiates, stimulants, or hallucinogens. An example is MDMA (known as ecstasy), a substance related to the hallucinogens and amphetamines. Designer drugs frequently contain contaminants or other impurities.