How are we supposed to know what rate we need to place our Oxygen machine meter on at night . Not knowing how low our Oxygen goes while sleeping. I lost so much sleep last night , kept waking to try to get a reading, everytime I took it, it was 98 99 . And only had it on 2 liters. Does it change as soon as your eyes open? Another dumb question after 2 years and 3 months im asking all these questions