Denne beskriver hvordan blodkarenes respons på CO2 er dårligere om morgenen, og det er derfor det skjer flere slag og slikt om morgenen. Den nevner mange interessante prinsipper. Bl.a. at lavere vasomotor respons (på CO2) gir mindre oksygen til hjernen. Og at i opptil 20 sekunder etter en 20 sekunder holdning av pust (etter utpust) øker fortsatt oksygenmengden og blodgjennomstrømningen i hjernen. Nevner også at siden blodkarene i hjernen reagerer dårligere på CO2 om morgenen blir det lett at pusten over- eller underkompenserer, så pustemønsteret blir uregelmessig om morgenen. Spesielt om man har underliggende faremomenter som hjerte/karsykdommer.

http://jap.physiology.org/content/102/5/1891

Furthermore, our results suggest that morning cerebral tissue oxygenation might be reduced as a result of a decreased cerebrovascular responsiveness to CO2 or other factors, leading to a higher level of desaturation.

Our data indicate that the cerebrovascular reactivity to CO2 in healthy subjects is significantly reduced in the morning and is strongly associated with an augmented ventilatory response to CO2. It is likely that this reduction in MCAV CO2 reactivity, by reducing blood flow through medullary respiratory control centers, increases both the arterial-brain tissue PCO2 difference and the H+ concentration presented to the central chemoreceptor(s) (11, 44). In effect, it appears the brain tissue is more susceptible in the morning to changes in arterial PCO2, which could increase the likelihood of ventilatory overshoots and undershoots.

However, as was the case with the hypercapnic challenge, subjects holding their breath in the morning experienced a significantly blunted increase in MCAV compared with evening, likely a result of a reduced cerebrovascular responsiveness to CO2.

In conclusion, our results suggest that early morning reductions in cerebrovascular CO2 reactivity strongly influence the magnitude of the ventilatory response to CO2. This may have significant implications for breathing stability, increasing the chances of periodic breathing in the morning in patients with additional risk factors. The early morning reduction in cerebral oxygenation with hypercapnic challenge, mild hypoxemia, or during apnea may be a contributing factor in the high prevalence of early morning stroke. Whether differences in the responses of CBF, oxygenation, or V̇E to CO2challenge are associated with other risk factors for stroke, such as gender or age, remains to be elucidated.