The participants were recruited from Samsung Medical Center, between December 6, 2021, and October 15, 2023. The study participants were recruited through both online and offline advertisement. The inclusion criteria were young and healthy adults aged 20 to 39. The exclusion criteria were 1) an inability to complete the questionnaires by intellectual disability, major neurocognitive disorder, severely impaired schizophrenia, 2) current severe episode of major depressive disorders or manic episode, 3) substance or alcohol abuse history within 1 years, 4) medication or medical illness affecting sleep or arousal, 5) shift-work, 6) smoking more than 6 months including last month, and 7) severe sleep disorders (e.g., apnea-hypopnea index score ≥30 or periodic limb movement during sleep index score ≥50).

Initially, 50 agreed to participate in the current study. Eleven did not complete the study, since they withdrew the consent. Finally, 39 (23.74±2.87 years old, 19 males and 20 females) completed the whole study procedures. Among them, 14 (25.00±2.94 years old, 7 males and 7 females) reported the presence of significant insomnia, while 25 (23.04±2.64 years old, 12 males and 13 females) did not. There were no significant differences in age or sex between those with and without insomnia.

All study procedures were carried out following the 1964 Declaration of Helsinki as revised in 2013. This study protocol was approved by the Institutional Review Board of Samsung Medical Center (protocol code 2021-09-126). All participants provided informed consent.

The Insomnia Severity Index (ISI) [11,12] was used to assess the severity of insomnia and screen the presence of insomnia. ISI is a 7-item self-report questionnaire used to assess for initial/maintenance/terminal insomnia, satisfaction with sleep, daytime function impairment, noticeability of insomnia, and worries about sleep. Each item is rated from 0 to 4. Total scores ≥8 indicate presence of significant insomnia in the current study.

The Profile of Mood States (POMS) is a 65-itme self-report questionnaire to measure transient state of various moods [13,14]. The POMS consists of 6 factors representing different moods (tension, depression, anger, vigor, fatigue, and confusion). Each item is rated from 0 to 5. Since it is designed to measure short-term change of mood status, it is appropriate to measure the short-term effect of the intervention on mood. In the current study, POMS was used to compare the emotional state before and that after sleep.

Attention was assessed by the computerized Comprehensive Attention Test (CAT). The CAT is a modified continuous performance test. The reliability and validity of the CAT have been reported to be acceptable [15]. The CAT has been used to study attention deficit in individuals with psychotic symptoms, sleep disturbances, or suicidal ideation [16-18]. Among 6 subsets in the battery of CAT, the sustained attention and the divided attention task were selected for the current study. The sustained attention can be defined as the ability to focus on a specific task for continuing time. Various shapes are presented in the computer screen every 2 seconds for 10 minutes. Participants are requested to respond to all shape stimuli but the X-shape. The task measures the capacity to inhibit responses to the specific stimuli under conditions of sustained attention. The divided attention is the multitasking ability to focus on multiple tasks or multiple task demands simultaneously. Auditory and visual stimuli are presented simultaneously every 2 seconds for 3 minutes 20 seconds in the divided attention task. Participants are requested to respond when auditory or visual stimulus is the same one with preceding stimuli. The numbers of omission errors (OEs) and commission errors (CEs) were calculated for both attention tasks. OEs, representing attention, are defined as failures to respond to the target. CEs, representing impulsivity, are defined as incongruous responses to the non-target. Additionally, the reaction times (RT), and the corresponding standard deviation of the RT for correct responses were calculated.

The bed used in the current study was Mongata Sway Bed (Mongata Inc., Seoul, Korea). The Mongata Sway Bed is an electric bed that moves once every 4 seconds in the head-to-foot direction. It moves up and down to 10 cm, and the angle is inclined by about 3 degrees. Movement of the Mongata Sway Bed can be controlled with the remote control.

The participants were asked to sleep 3 nights in the Mongata Sway Bed with different modes for each night. Nocturnal polysomnography were also done during sleeping in the Mongata Sway Bed. The three different modes of sleeping bed were 1) motionless bed condition, 2) continuously rocking bed condition, and 3) before-sleep rocking bed condition. In the motionless bed condition, participants slept without rocking function for all night. In the continuously rocking bed condition, rocking started when the lights was off and stopped when the participants waked up in the morning. In the before-sleep rocking bed condition, rocking also started when the light was off, but rocking was stopped when sleep onset was found in the polysomnographic findings.

The order of the sleeping bed conditions was randomly assigned. Interval from the first experimental night to the next experimental night for each participant was more than 1 week. Interval from the first experimental night to the last (third) experimental night for each participant was less than 8 weeks.

Trip to large jet lag areas was prohibited 1 week before experimental sleep. Napping more than 60 minutes, drinking alcohol, drinking coffee or other caffeine drinks more than 5 cups a day, and sleep deprivation more than 3 hours compared to usual sleep duration were prohibited 3 days before experimental sleep. When participants failed to keep the above precautions, the participant were excluded or experimental sleeps were postponed or excluded. POMS for mood change and CAT for attention change were conducted in the evening before sleep in each experimental night. POMS and CAT were conducted again after waking up in each morning.

Paired t-test was used to compare mood and attention before and after sleep in rocking bed (for both continuously rocking and before-sleep rocking condition). Repeated measured analysis of variance was performed to compare three conditions (continuously rocking, before-sleep rocking, and motionless conditions) in mood and attention changes after sleep in bed. In order to explore the effect of rocking bed in those with insomnia, those whose ISI score was high were selected. The same statistical analysis was repeated for this subgroup with insomnia. The analysis was conducted using SPSS version 27.0 software (IBM Corp., Armonk, NY, USA). A p-value ≤0.05 was considered significant.

No participants reported any critical adverse effects due to sleeping in continuously rocking bed. After sleeping in the continuously rocking bed condition, depression (t=2.48, p<0.05), tension (t=3.90, p<0.001), anger (t=2.18, p<0.05), fatigue (t=2.66, p<0.05), and confusion (t=3.77, p<0.001) in POMS were significantly reduced (Table 1). Vigor did not change after sleeping in the continuously rocking bed condition. After sleeping in the continuously rocking bed condition, RT (t=2.49, p<0.05) and SD of RT (t=2.06, p<0.05) in the sustained attention tasks were increased, and SD of RT (t=8.82, p<0.001) in the divided attention tasks were decreased (Table 2).

No participants reported any critical adverse effects due to sleeping in before-sleep rocking bed. After sleeping in the before-sleep rocking bed condition, depression (t=2.76, p<0.01), tension (t=3.14, p<0.01), fatigue (t=2.23, p<0.05), and confusion (t=2.82, p<0.01) were significantly reduced (Table 3). Anger and vigor did not change after sleeping in the before-sleep rocking bed condition. After sleeping in the before-sleep rocking bed condition, OEs (t=2.14, p<0.05) in the divided attention tasks was reduced (Table 4).

There were no significant differences in after-sleep mood changes in any subcategories in POMS between three different bed conditions, i.e., motionless bed condition, continuously rocking bed condition, and before-sleep rocking bed condition (Table 5). In addition, there were no significant differences in after-sleep attention changes measured by CAT between three different bed conditions (Table 6).

However, in the subgroups with insomnia, there were differences in after-sleep mood and attention changes between three different bed conditions. There were significant differences in after-sleep improvements of depression between bed conditions. The improvement of depression after sleep were larger when participants with insomnia slept in continuously rocking bed or before-sleep rocking bed compared to when they slept in motionless bed (Table 7). In addition, there were significant differences in after-sleep improvements of divided attention between bed conditions. Reductions of RT in divided attention after sleep were larger when participants with insomnia slept in continuously rocking bed compared to when they slept in before-sleep rocking bed (Table 8). Reductions of SD of RT in divided attention after sleep were larger when participants with insomnia slept in continuously rocking bed compared to when they slept in motionless bed or before-sleep rocking bed (Table 8).