Relevance Between Reduction of SpO₂ and Parasympathetic Nervous Activity During Sleep

Sleep apnea syndrome (SAS) is associated with cardiopulmonary and dental nasopharyngeal diseases. The blood oxygen saturation (SpO₂) detected using pulse oximetry is a diagnostic screening method for SAS. SAS severity is assessed using the oxygen desaturation index (ODI), which measures the number of times the SpO₂ decreases by more than 3% for longer than 10 s. This study investigated the association between reduced SpO₂ and parasympathetic nervous system activity (PSNA) during night sleep in young adults (n = 63; aged 20-32 years). Changes in PSNA were measured every minute for 24 h during a free-moving day using an ActiveTracer accelerometer. Pulse oximetry was performed simultaneously during sleep. All participants had significantly lower PSNA when the SpO₂ decreased by 3% or more than when it did not. There were no significant differences in PSNA when an ODI cutoff of 5 events/h was used. However, participants with an ODI >5 events/h had significantly lower PSNA during normal SpO₂ than those with an ODI <5 events/h, suggesting an association between SpO₂ desaturation and PSNA decline during sleep. A high ODI may indicate reduced PSNA levels during sleep, affecting sleep efficiency. Treatment aimed at reducing the ODI may improve sleep quality, even in young adults.