Breathing becomes shallower and lighter when falling asleep

Key Points

• CO2 increases significantly prior to or simultaneously with sleep onset

• Breathing becomes shallower with sleep onset

• Breathing rate does not change with sleep onset

The Breathing Diabetic Summary

We have reviewed several studies on sleep and breathing (here, here, and here).  However, we have not looked at how breathing changes as we fall asleep.  This study sheds light on central nervous system changes occurring during sleep onset.

Four women and 8 men that had no history of respiratory or sleep disorders were studied. They were instructed to relax in a reclined bed in the dark and go to sleep for ~1 hour.  Five of the subjects also were asked to relax in the bed, but stay awake for 45 minutes for control measurements.

(This is a very small sample size, so let’s remember that as we go through the results.)

Measurements of alveolar CO2 and chest and abdominal breathing motions were taken to reflect central nervous system changes during sleep onset.  Breathing rate also was measured.  The subjects went through 3 separate sessions to acclimate to the laboratory setting.

During all three sessions, when the participants fell asleep, their CO2 rose significantly and their breathing became shallower.  The significant increase in CO2 was not associated with a change in breathing rate.  This implies that breathing became not only shallower, but also lighter.

In every case, CO2 rose and breathing became shallower either prior to or simultaneously with falling asleep.  And because breathing rate did not change, the rise in CO2 indicates that the subjects were breathing less.

The close correlation between CO2, shallow breathing, and sleep onset indicates that breathing patterns might be a reliable indicator of drowsiness in a person.

Abstract

This study provides a systematic examination of factors that may contribute to respiratory changes associated with sleep onset. The electroencephalogram, alveolar CO2 tension, patterns of abdominal and thoracic respiratory movements, and respiratory rate were measured in three sessions each on 12 normal subjects as they fell asleep, and also on 5 of them as they lay awake. Nonintrusive respiration measurement devices were used. Resting awake CO2 tension was found to increase significantly across sessions. In addition, CO2 tension was significantly higher during stages 1 and 2 of sleep than during wakefulness on days 2 and 3. There was also a shift from relatively greater abdominal expansion toward relatively greater thoracic expansion with sleep onset. None of these changes occurred when subjects remained awake during a session. We conclude that changes in respiration with sleep onset cannot be accounted for solely by changes due to habituation, merely lying quietly, or the effects of the measuring devices. Rather, they appear to be caused by a central interaction between centers controlling the level of wakefulness and those controlling respiration.

Journal Reference:

Naifeh KH, Kamiya J.  The nature of respiratory changes associated with sleep onset.  Sleep.  1981;4(1):49-59.

Your breathing is shallow and irregular for 1/3 of your life

Key Points

• Breathing volume decreases between 6% and 16% during sleep

• Breathing is shallow and irregular during sleep

• We experience relative hypoxia and hypercapnia during sleep

The Breathing Diabetic Summary

To understand sleep-related breathing disorders, we first have to understand normal breathing during sleep.  That was the goal of this study.

Experiments were conducted with 19 subjects (8 males, 11 females) that had no history of sleep complaints.  Additionally, they were all nocturnal sleepers.  The researchers studied them between 10 PM and 7 AM.  They studied one subject on 3 nights, 9 subjects on 2 nights, and 9 subjects on 1 night.

Baseline measurements were obtained while the patients were lying in bed either before falling asleep or after waking up (using EEG-confirmed wakefulness).  Theses recordings were subsequently averaged to produce the “awake” value. 

For measurements of breathing during different sleep stages, the subjects had to stay in that sleep stage continuously for at least 2 minutes.  Additionally, there could not be any detectable leaks within the breathing mask they were wearing.

Comparison of the awake versus sleeping parameters revealed that breathing volume reduced significantly during sleep.  For non-REM sleep, breathing volume decreased between 6% and 8%.  During REM sleep, ventilation reduced by ~16%.  Interestingly, the breathing rates of these subjects were slightly faster during sleep than while awake, suggesting that breathing becomes shallower during sleep.

Because the participants were breathing less, they became significantly more hypoxic (low O2) and hypercapnic (high CO2) while asleep compared to while awake.

The researchers used this information, along with assumptions regarding lung dead space and dead space due to the breathing mask, to estimate the change in gas exchange occurring in the lungs.  These calculations revealed a reduction in gas exchange between 19% and 39% during sleep, helping explain why the participants experienced hypoxia and hypercapnia.

Lastly, during non-REM sleep, breathing rates were somewhat regular (although a few patients still showed irregular rates during non-REM).  In REM sleep, all participants exhibited shallow and irregular breathing patterns. 

Overall, these results show that breathing volume is reduced during all stages of sleep. The greatest reductions occur during REM sleep, which is also when breathing rate is the most irregular and unstable. The reduction in breathing leads to relative hypoxia and hypercapnia. Interestingly, these breathing patterns are normal and are part of the natural physiological changes our bodies makes during sleep.

Abstract

Respiratory volumes and timing have been measured in 19 healthy adults during wakefulness and sleep. Minute ventilation was significantly less (p less than 0.05) in all stages of sleep than when the subject was awake (7.66 +/- 0.34(SEM) 1/min), the level in rapid-eye-movement (REM) sleep (6.46 +/- 0.29 1/min) being significantly lower than in non-REM sleep (7.18 +/- 0.39 1/min). The breathing pattern during all stages of sleep was significantly more rapid and shallow than during wakefulness, tidal volume in REM sleep being reduced to 73% of the level during wakefulness. Mean inspiratory flow rate (VT/Ti), an index of inspiratory drive, was significantly lower in REM sleep than during wakefulness or non-REM sleep. Thus ventilation falls during sleep, the greatest reduction occurring during REM sleep, when there is a parallel reduction in inspiratory drive. Similar changes in ventilation may contribute to the REM-associated hypoxaemia observed in normal subjects and in patients with chronic obstructive pulmonary disease.

Journal Reference:

Douglas NJ, White DP, Pickett CK, Weil JV, Zwillich CW.  Respiration during sleep in normal man.  Thorax.  1982;37(11):840-844.