When we sleep, our breathing changes due to normal biological processes that affect both our respiratory and muscular systems.
As our sleep deepens, our minute ventilation continues to decrease, reducing by 13% in the second NREM stage and by 15% in the third.
For example, a study of 19 healthy adults revealed that the minute ventilation in NREM sleep was 7.18 liters/minute compared to 7.66 liters/minute when awake.
Irregular breathing with sudden changes in both amplitude and frequency at times interrupted by central apneas lasting 10–30 seconds are noted in Rapid Eye Movement (REM) sleep.
Tidal volume has also been shown to be increased, decreased or unchanged by quantitative measures in REM sleep.
In a study of 19 healthy adults, the minute ventilation in REM sleep was 6.46 +/- 0.29(SEM) liters/minute compared to 7.66 +/- 0.34 liters/minute when awake.
This is due to REM related supraspinal inhibition of alpha motoneuron drive and specific depression of fusimotor function.
This decrease in intercostal muscle activity is primarily responsible for hypoventilation that occurs in patients with borderline pulmonary function.
Changes in altitude cause variations in sleep time (reduced to 0% up to 93%), as shown in a study that examined people at sea level and Pikes Peak (4300 meters).
Each pause in breathing, called an apnea, can last for several seconds to several minutes, and may occur 5 to 30 times or more in an hour.