Assuming that sleeping with head under blanket increases CO2 levels, you can find plenty of studies on the more immediate effects of the latter, e.g.,
It has been shown that when bedroom air quality was improved in these experiments:
- Subjects reported that the bedroom air was fresher.
- Sleep quality improved.
- Responses on the Groningen Sleep Quality scale improved.
- Subjects felt better next day, less sleepy, and more able to concentrate.
- Subjects' performance of a test of logical thinking improved.
As for sleep apnea, the longer-term associations between that and brain function have been studies (because it occurs naturally)
people with sleep apnea tended to develop memory problems and other signs of mild cognitive impairment (MCI)
As to directly answer your question, I think I know where the 23% came from:
What percentage of dementia overall could be attributed to head covering while sleeping? Considering those 60 years of age and older, comparing those who do not practice head covering and were at a middle or late stage of dementia (assuming these to have suffered dementia due to causes other than head covering) to those that practice any level of head covering and who were at a middle or late stage of dementia (causes other than head covering plus head covering), 23% were attributed solely to head covering. If we consider that this percentage could also be attributed to all levels or stages of dementia, we can assume that 23% of all cases of dementia are due to head covering while sleeping.
It's a self-published (thus not peer-reviewed) survey conducted by an elementary school teacher.
There's not much in the way of direct studies of the effects of blankets on even breathing, let alone longer-term effects... I found one pretty old (1972) and small-scale experimental study, probably conducted at a US army facility given the author's affiliation, which was published in JAMA (as very, very short note):
Methods.— The effect on the inspired
oxygen and carbon dioxide concentrations
was studied in
five healthy men,
25 to 30 years old. Each volunteer
rested quietly though not sleeping, in
the left lateral decubitus position on a
sheet-covered mattress. The head was
covered for ten minutes by one sheet
and one standard AUS blanket weighing
3 to 4 lb and then by one sheet
and two blankets for an additional
ten minutes. All coverings were
tucked under the mattress except the
headward edge which was laid flat on
the top surface. Samples of air from
under the covers were obtained at
one, five, and ten minutes from a
catheter placed adjacent to the mouth
of the subject. The subject held his
breath at the end of inspiration during
the sampling interval. The barometric pressures varied between 757
and 759 mm Hg during the study.
Results.— Before covering the head,
the oxygen concentration was 20.7%
to 20.9%, and the carbon dioxide concentration was 0.4% to 0.5%. Upon
covering the head with one or two
blankets, the mean oxygen percentage rapidly decreased to 18% to 19%
and the mean carbon dioxide rose to
1% to 2% within one minute and
thereafter remained stable. Hypoxia
as low as 16.5% oxygen, and hypercapnia
as high as 4.1% carbon dioxide
was obtained beneath the covering of
To ascertain the frequency of this
habit, 100 healthy individuals, 41
women and 59 men with a mean age
of 32 years were asked whether they
cover their heads before falling
asleep. The survey revealed that 11 of
them, two women and nine men, with
a mean age of 29 years, covered their
heads with one or more blankets before going to sleep. Most of them (7 of
11) covered their heads every night or
every other night.
The author went on say that the changes were insignificant in healthy individuals, but relevant in those with "borderline compensated oxygen delivery system such as congestive heart failure" and that "hypercapnia may be harmful in that carbon dioxide may induce cardiac arrhythmias or depress myocardial contractility in predisposed individuals."
Also, I looked for studies on whether hypercapnia can cause sleep apnea, but I wasn't able to find that. What I did find was that some but not all people with obstructive sleep apnea also exhibit daytime hypercapnia.
In sleep apnea patients, the chronic hypercapnia develops adaptive mechanisms to diminish the hypoxic negative effects.[citation] Plausible explanations to this effect are hyperventilation, increases in the ventilation–perfusion relationship, improvements in tissue oxygen delivery changing the hemoglobin oxygen affinity, increases in CBF due to vasodilation, and so forth. Brain oxygenation can improve when hypercapnia and hypoxemia are combined; nevertheless, this is a controversial topic.[citations]
And regarding ASSB (accidental suffcation and strangulation in bed), it's generally considered a risk factor for infants, but dimishes drasticly after 6 months of life.
So I think that discounting the self-published survey, most of the inferences in the original question here can be chalked up to jumping to conclusions from seemingly plausible mechanisms, but not supported by available evidence.