Sleep-waking pattern and body temperature in hypoxia at selected ambient temperatures.

We studied the effect of mild hypoxia (15% O2) and low ambient temperature (Ta = 15 degrees C) on the rat's sleep-waking pattern (SWP) and maximum-minimum core temperature (max-min Tb). Mild hypoxia at neutral Ta (29 degrees C) disrupted the SWP in the same way as low Ta during normoxia: both affected the pattern of frequency of state changes (P less than 0.01), not the pattern of epoch durations. Mild hypoxia and low Ta together caused a degree of disruption of the SWP which was the sum of each alone, i.e., additive. Although both mild hypoxia and low Ta significantly depressed max-min Tb, low Ta exerted a greater effect than mild hypoxia. Together they further depressed max-min Tb in an additive way. We conclude that mild hypoxia disrupts the rat's SWP independent of central thermoregulatory mechanisms at neutral Ta, that the effects of mild hypoxia and low Ta on the SWP are additive at the stimulus levels used, and that Ta, not inspired O2, determines Tb.