Obstructive sleep apnea is normally at its worst when the patient is________________.

Obstructive sleep apnea (OSA) tends to be most severe during REM sleep, particularly when the patient is supine, or on their back. This is due to the unique characteristics of REM sleep, where muscle atonia occurs, leading to decreased tone in the upper airway muscles. As a result, the airways are more likely to collapse during this phase, allowing for the distinct patterns of apnea and hypopnea associated with OSA.

In addition, lying on the back exacerbates this condition because gravity can contribute to the collapse of the upper airway structures. These factors together create an environment where breathing can become significantly impaired, leading to observable episodes of apnea and oxygen desaturation.

Other positions or stages of sleep do not present the same level of risk for these complications. For example, patients may experience less obstruction when positioned on their sides or while sitting up due to improved airway patency in those positions. Deep sleep may also bring about some level of airway collapse, but it does not typically reach the severity seen during REM sleep. Thus, REM sleep in the supine position is the critical combination that leads to increased severity in obstructive sleep apnea.