[thedoc (OP)]

Karen asked the Naked Scientists:
   My mother wakes up at night when her heart can't supply enough oxygen into her brain. Are people more alert when their brain cells lack oxygen?
What do you think?

[alancalverd]

Far from it! Nitrogen or altitude hypoxia is extremely pleasant, like being slightly drunk and unworried. It's my chosen method of suicide.

More likely is that Karen's mum is being woken by apnoea - where the throat walls relax and cut off air intake - or excessively shallow breathing leading to an increase in blood carbon dioxide, which triggers a panic response.