scientific literature have been made by Leaning, Schacter, Richardson and Mavromatis. Research Self-observation (spontaneous or systematic) was the primary tool of the early researchers. Since the late 20th century, this has been joined by questionnaire surveys and experimental studies. All three methods have their disadvantages as well as points to recommend them. Naturally, amnesia contributes to the difficulty of studying hypnagogia, as does the typically fleeting nature of hypnagogic experiences. These problems have been tackled by experimenters in several ways, including voluntary or induced interruptions, sleep manipulation, the use of techniques to "hover on the edge of sleep" thereby extending the duration of the hypnagogic state, and training in the art of introspection to heighten the subject's powers of observation and attention. Techniques for extending hypnagogia range from informal (e.g. the subject holds up one of their arms as they go to sleep, to be awakened when it falls), to the use of biofeedback devices to induce a "theta" state – produced naturally the most when we are dreaming – characterized by relaxation and theta EEG activity. Another method is to induce a state said to be subjectively similar to sleep onset in a Ganzfeld setting, a form of sensory deprivation. But the assumption of identity between the two states may be unfounded. The average EEG spectrum in Ganzfeld is more similar to that of the relaxed waking state than to that of sleep onset. Wackerman et al. conclude that "the Ganzfeld imagery, although subjectively very similar to that at sleep onset, should not be labelled as 'hypnagogic'. Perhaps a broader category of 'hypnagogic experience' should be considered, covering true hypnagogic imagery as well as subjectively similar imagery produced in other states." See also References Bibliography Further reading Sacks, Oliver (2012). "On the Threshold of Sleep". Hallucinations. External links "Hypnagogic and hypnopompic hallucinations: pathological phenomena?" in the British Journal of Psychiatry "Hypnagogia" by Gary Lachman in Fortean Times Sleep disorders Sleep physiology Lucid dreams Hallucinations