Sleep, respiratory physiology, and nocturnal asthma.

The nocturnal worsening of asthma is a common feature of this disease that recently has received extensive investigation. Most recent efforts have focused on the role of circadian biorhythms that could promote a nocturnal increase in airway inflammation, leading to a subsequent increase in airflow obstruction and asthma symptoms. However, definitive studies remain lacking. As discussed in this review, there is also substantial evidence that sleep itself may play a direct role in the nocturnal worsening of asthma. Potential mechanisms for such a sleep-related effect could include the supine posture, alterations in sympathetic and parasympathetic "balance," sleep-associated reductions in lung volume, intrapulmonary pooling of blood, and sleep-associated upper airway narrowing, both with and without snoring and obstructive sleep apnea (OSA). These potential contributors to this troublesome phenomenon deserve further consideration when investigating mechanisms of nocturnal asthma.