Systematic decrease of slow‐wave sleep after a guided imagery designed to deepen sleep in low hypnotizable subjects

Slow‐wave sleep is one of the most important restorative components of sleep and central for our health and cognitive functioning. Although the amount of slow‐wave sleep depends on sleep drive, age and other factors, also the pre‐sleep mental state might influence sleep depth. We had shown that a pre‐sleep hypnotic suggestion to sleep more deeply increased slow‐wave sleep duration in hypnotizable subjects. In contrast, low‐hypnotizable participants decreased sleep depth after this intervention. A possible reason might be an aversion to and active resistance against hypnosis. To overcome this potential opposition, we introduced the procedure as ‘guided imagery’. We replaced the hypnotic induction by a breathing relaxation. Importantly, the suggestion ‘to sleep more deeply’ remained identical. We expected that these changes would make it easier for low‐hypnotizable subjects to benefit from the suggestion. In contrast, young healthy low‐hypnotizable participants did not show positive effects. Similar to our previous studies, they exhibited a reduced slow‐wave sleep duration after the intervention. Additionally, the ratio between slow‐wave activity and beta band power decreased. Subjective sleep quality remained unaffected. Our results indicate that suggestions to sleep more deeply result in decreased sleep depth in low‐hypnotizable participants regardless of the mental technique (guided imagery versus hypnosis). Thus, the aversion against hypnosis per se cannot explain the detrimental effect of the intervention on slow‐wave sleep in low‐hypnotizable subjects. The results support the notion that our mental state before sleep can influence subsequent slow‐wave sleep. However, the mechanisms of the contradictory decrease in low‐hypnotizable subjects remain unknown.