Nocturnal decrease of arterial oxygen content—hidden stimulus for erythropoietin secretion at altitude

To the Editor:
In their investigation in male mountaineers in the Andes,
Tannheimer et al.1 showed that arterial oxygen saturation
(SaO2) during undisturbed night sleep increased from a nadir
during the first half to a higher value during the second half and
a maximum in the morning after wake-up. The difference
diminished with acclimatization and ascent from 3050 m to
5630 m of altitude during 19 days but remained significant.
Unfortunately, the time of day was not communicated.
Measurements of diurnal variation in SaO2 at altitude are
rare. Apparently, investigations in sojourners have been published only twice (reviewed in1
). In addition to altitude acclimatization physical training by repeated ascents might play a role
in the improvement of SaO2.