Timing, Sleep, and Respiration in Health and Disease - Progress in Molecular Biology and Translational Science - page 204

intrusions lasting 10-30 s or more. These breathing irregularities are linked
to the occurrence of bursts of rapid eye movements for which REM sleep is
named. 161-163
During sleep, there is a reduction in excitatory cortical input to the upper
airway muscles resulting in decreased tone and reduced patency. 164-167 This
is especially problematic during early NREM (stage N1) and REM, sleep
stages in which breathing is particularly unstable. 168 Arousal leads to an
increase in excitatory input to the upper airway muscles and improvement
in airway patency. 169,170
Interestingly, there has been some controversy over the effects of sleep
deprivation on the HCVR with some studies suggesting that sleep depriva-
tion reduces the HCVR and others showing that it does not. With the aid of
the constant routine protocol, Spengler and Shea most convincingly dem-
onstrated that sleep deprivation has no effect on the HCVR. 88
As discussed, a major purpose of breathing is to maintain CO 2 /pH
homeostasis in order to preserve respiratory drive and ensure adequate oxy-
gen availability to tissues. This is certainly true during sleep. During sleep,
sensitivity of the CO 2 regulating mechanism is reduced. As such, much
higher concentrations of CO 2 are required to induce a small increase in ven-
tilation. 171 During sleep, the decreased V E allows the CO 2 level to rise to the
sleep set point which is higher than it is during wakefulness ( Fig. 8.4 ). Dur-
ing brief arousals from sleep, there is an increase in ventilation that serves to
reduce the CO 2 level to the wakefulness set point. 172 In a similar manner to
hypercapnia, there is also a state-dependent variation in the ventilatory

A B

Alert
Quiet
Waking

20

450
400
350
300
250
200

Stage 1

15

WAKE

Stage 2

NREM
REM

10

Stage 3
Stage 4

5

0 6 12 18 24

Zeitgeber time (h) 0

30 35 40

P A CO 2 (mm Hg)

45 50

Figure 8.4 Vigilance-state dependence of breathing. (A) Average ventilation over 24 h
in Wake, NREM, and REM as indicated in rats housed in a 12:12 light:dark cycle. (B) State-
dependent variation in the sensitivity of the hypercapnic ventilatory response. (A) Red-
rawn with permission from Ref. 150 and (B) from Ref. 171 .