Yes, when a patient with untreated obstructive sleep apnea has a respiratory event during sleep whereby their upper airway partially collapses (hypopnea) or completely collapses (apnea), the brain can respond to the fall in oxygen or rise in carbon dioxide with a microarousal. This is detected during a sleep study by monitoring the brain waves. A microarousal doesn’t necessarily mean that the patient becomes fully conscious or awake. Microarousals can shift the patient’s stage of sleep from deep sleep (REM sleep and slow-wave sleep) to a lighter stage of sleep. Over the course of one night of sleep, a patient with untreated sleep apnea can have numerous microarousals which prevent the patient from staying in a sustained phase of deep sleep. If left untreated, a patient may experience the equivalent of chronic partial sleep deprivation.