Much anecdotal evidence has been put forward in favour of sleep learning, although even the anecdotal evidence varies considerably. It is possible that some of the effects of classical conditioning may be extended by sleep in some limited ways. But, in scientific terms, the concept of sleep learning has been conclusively disproved in recent years, and it is widely considered a pseudoscience. Repeated attempts to teach vocabulary, foreign languages and even lists of items to sleeping subjects in controlled experiments have all failed miserably. It appears that, in order for new learning to occur (as opposed to the consolidation and reconsolidation of existing knowledge and memories) , alpha wave brain activity is needed at the same time as the stimulus material, and, although this may occur just before sleep or upon awakening from sleep, it does not occur during normal non-REM or REM sleep.
Interestingly, some recent research from Germany has indicated that, although sleep learning in the traditionally-understood sense is not a reality, there may be a possibility of boosting memory formation by auditory stimulation, playing sounds at certain specific times during the sleep cycle. When brain waves are generating the slow oscillating rhythm associated with memory consolidation during sleep, then auditory stimulation in sync with this rhythm has been shown to amplify and extend the oscillations, thereby improving memory retention. Short, unintrusive, low-volume bursts of sound are all that are needed to achieve this, the more important aspect being the timing (to coincide with brain wave activity).
As well as distinguishing between sleep learning and the normal reconsolidation and analysis of daytime learning (which is widely agreed to be one of the major functions of sleep - see the section on Memory Processing and Learning), sleep learning should also be distinguished from hypnosis or self-hypnosis. Contrary to popular misconception, a hypnotic trance is not a form of unconsciousness or sleep, but hypnotic subjects are actually fully awake, albeit with increased focus and concentration, decreased peripheral awareness and a tendency to block out all external sources of distraction. Although hypnosis and hypnotic suggestion does alter the way that the brain processes information to some extent, and it may be effective in pain control and treatments of some other conditions, accelerated learning through hypnotism-based techniques like neuro-linguistic programming remains scientifically unvalidated and is not considered part of mainstream academic psychology.