For more information, see Kaur et al., Neuron 96/5, http://www.cell.com/neuron/fulltext/S0896-6273(17)30977-7.
Kaur et al. identify calcitonin gene-related peptide neurons in the parabrachial nucleus as a key switch for waking up the brain in response to elevated CO2. This switch provides a target for preventing sleep disruptions in obstructive sleep apnea.