As we here applied the same study design as in those previous experiments, we were interested if the effect of music on sleep also depends on expectations and self-suggestions.
A similar pattern of results occurred in a conceptual replication of this study in elderly women 30. No enhancement of SWS was observed for low suggestible females.
RELAXING MUSIC DEEP SLEEP MUSIC SERIES
In a series of studies 29, we have shown that hypnotic suggestion to sleep deeper increases deep SWS and slow wave activity (SWA) in high suggestible healthy participants. It was even shown that suggestions impact on memories, beliefs and behaviors 28. Also, a suggestion to not attribute meaning to Stroop words reduced the Stroop effect in highly suggestible subjects irrespective of whether a hypnotic state was established before or not 27. Suggestibility accounts for strong individual differences in the effects of hypnotic interventions, but also outside of hypnosis, like for instance “placebo” effects 25 (but see 26). Suggestibility describes the ability of a person to respond to suggestions in terms of perceptual, cognitive, neural and bodily processes 24. One possible factor being involved in individual differences in the effects of non-pharmacological interventions on objective sleep may be suggestibility. This paper points towards the possibilities that the effects of music on objective sleep parameters might depend on certain individual differences. However, further, more fine grained analyses of sleep-related oscillations are missing in this study. 23 reported effects of music on objective sleep parameters: One hour of listening to music after subjects went to bed significantly decreased the amount of stage N2 sleep, and increased deep SWS only in a subgroup of participants with long sleep latencies. 22 did not observe any positive effects of music playing when lying in bed on objective measures of sleep onset, total sleep time, sleep interruption and sleep efficiency, in spite of positive effects of music on subjective sleep quality. For example, Lazic and Ogilvie 21 did not find differences in polysomnographic measures when subjects in a within design either listened to music, tones or neither tones nor music after lights off until continuous sleep was observed. In contrast to subjective sleep quality, empirical findings on the effects of music on objectively measured sleep are scarce and inconsistent.
In sum, the positive effects of listening to music on subjective ratings of sleep quality are well established across different age groups including both healthy participants and patients. Also in older women, two further studies reported shorter sleep onset time, less nocturnal awakenings and better sleep quality and satisfaction as measured with a questionnaire and sleep logs when listening to music when going to bed 19, 20. Positive effects of listening to music on subjectively evaluated sleep were found in toddlers when listening throughout the naptime, preschool children and young adults listening 45 minutes at naptime and/or bedtime 16, 17, 18. In 20 included trials, another meta-analysis showed positive effects of music based interventions before bedtime on PSQI scores, sleep onset latency and sleep efficiency in patients suffering from primary insomnia 15.
Similarly, sedative music while resting could effectively improve subjectively rated sleep in patients with sleep complaints 14. In a meta-analysis based on six included studies in a total of 314 patients, Jespersen and colleagues reported that music helped to improve subjective sleep quality in insomnia patients 13. In a survey in over 500 patients with sleep disorders, over 50% reported to use music as sleep aid 12. In an online survey in a general population 62% (out of n = 651 respondents) stated to have at least once used music to help them sleep 11. Listening to music is a widely used tool to improve sleep. Consequently, researchers need to empirically validate the effectiveness of non-pharmacological and easy to implement tools to support healthy sleep. The use of sleep enhancing medicine is problematic, as its effectiveness decreases across time and may lead to addiction. However, sleep disturbances are highly common in our society 8, with increased prevalence in ageing 9 as well as among people at risk of or suffering from a psychiatric disorder 10. Sleep plays an important role for maintaining physical and mental health 1, 2, 3, 4, 5, and is critical for general well-being 6, 7.
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