low oxygen night time

hi
  my pulmo is putting me on a two week assesment on a nubiliser, as my oxygen drops to the lower 70s now, he wants to see if it makes my levels stable my oxygen, and my carbon dioxide as when they give me oxygen my dioxcide goes up as well, i sleep well at night but when i wake up i have a bad headache and feel ****, i also find i start to get sleepy in the day time, i have oxygen for doing things like walking a bit of gardening 4lts and no problem with that, but have to turn it off when resting or i think my dioxide goes when resting with oxygen, has anyone or know anyone who has this problem, and could a nubiliser put this right, the assesment he said i would have a diary and would have to put the numbers in, every so often
                                                  thanks  need some advice on this