What does the "arousal index" measure in polysomnography?

The arousal index is a crucial metric used in polysomnography to quantify sleep disturbances and assess sleep quality. It specifically measures the number of arousal events that occur per hour of sleep. An arousal event is defined as a shift in the sleep state that can be detected by changes in brain waves, typically monitored by an electroencephalogram (EEG), or changes in muscle tone or respiration.

Understanding the arousal index is vital for evaluating sleep architecture and determining the presence of sleep disorders, such as obstructive sleep apnea or insomnia. Higher arousal indices often indicate fragmented sleep, which can contribute to daytime sleepiness and other health issues.

In contrast, the total time spent awake during the study refers to a different aspect of sleep quality and does not specifically quantify arousal events. The time taken to fall asleep and sleep efficiency percentages focus on different metrics of sleep onset and overall sleep quality, but they do not provide a direct measure of arousal frequency during sleep as the arousal index does.