Suspension pillow reduces stress and improves nocturnal sleep.

The head of an adult human weighs approximately 5 kg. A heavy head is always supported by the neck, causing stiffness in the neck and shoulders. Stiffness results in physical and mental stress. A suspension pillow (SP; Hammock Pillow, M. I-Story, Japan) was devised to disperse the pressure on the neck while the user laid down on a mat. The SP has three pillars that suspend a hard cloth like a hammock. The head is held on the cloth. The SP is expected to decrease stiffness, relieve physical and mental stress, and improve the sleep quality. In the first experiment, we showed stress-relieving effect of the SP by measuring and analyzing fluctuation of heart beat interval (heart rate variability: HRV). HRV is indexed as LH/HF and signifies autonomic nervous system activity. The participant was instructed to lay the head on the pillow and exercise very lightly for 7 min to stretch the neck muscles and facilitate cardiovascular circulation. On the next day, he or she was asked to perform the same exercise without the SP. The experimental schedule was randomized among the participants (n=7). Heart beats were measured using a plethysmogram from the index finger. The LF/HF ratio decreased by 39% after exercise with the pillow. Statistical analysis also revealed significant activation of the parasympathetic nervous system and a stress-reducing effect of the SP. This indicates that the SP can be a tool for stress reduction. In the second experiment, we performed a sleep study to demonstrate the sleep-promoting effect of the SP. Middle-aged and older adults tend to explicitly complain of deteriorated sleep quality compared to young adults. Therefore, we recruited these participants for this experiment to demonstrate the sleep-promoting effect of the SP. Six participants (40-70 years old) participated in the experiment. They slept with the SP or a Japanese traditional and popular pillow stuffed with buckwheat chaff (JP) in their bedrooms for four to five consecutive nights. Data obtained during the first night with each pillow were excluded to avoid the first-night effect. The order of pillow usage was randomized among the participants. Heart beat and body movements were monitored using a plethysmogram on the index finger and an accelerometer placed around the wrist to measure nocturnal sleep quality. The results demonstrated that the SP improved sleep efficiency and increased the amount of deep sleep in NREM (non-REM sleep but decreased the counts and amount of nocturnal awakening. These results prove that the SP improves sleep quality in middle-aged and older adults. In the last experiment, we recorded polysomonography data from three young participants (21-23 years old) in an electromagnetically shielded room. We found that sleep latency was shortened significantly in the three young participants with the SP compared to JP. The findings obtained from three experiments revealed that the SP holds the head adequately, resulting in reduced stress by activating the parasympathetic nervous system and improving sleep quality.