You misunderstood some of what I was saying, point 1 specifically is not necessarily false. The point I was trying to convey is that rather than feeling good because you have no recorded apnea it is probably that you feel good because you had a good night of sleep with few disturbances and falsely flagged events leading to a low recorded AHI. That said I am guessing you also have some nights you feel good after AHI of 1 since AHI is an imperfect statistic and not an indicator of sleep quality.
PLM can be an issue and definitely can be triggered/worsened by apnea/breathing. This improving is a sign that CPAP has helped. PLM may also be being triggered by other things (like you say caffeine, bad night etc) or imo may be a symptom of underlying issue that is causing how you feel. PLM will often show up in your data because muscle activation and movement often does affect lungs. If you record your sleep with video you will find it pretty amazing to see how almost all of your flow rate blips and oddities are usually explainable to some physical phenomenon and not necessarily a breathing issue. PLM arousals will show up as arousal breathing, PLM without arousals will probably show up as periodic blips in flow rate. Sheepless is a member that suffers from PLM and has a thread on it with some examples of how it shows up in his data as well as other comments on treatment although I haven't read it all in detail.
http://www.apneaboard.com/forums/Thread-...g-movement
The tricky part of improving sleep is how complex it is and then when the main complaint is something like brain fog it becomes even more difficult because that isn't specific to sleep quality.
I myself have been on a journey for the past 4 years now to improve my sleep and health in general. At one point I figured/hoped it was all apnea but I have since realized that is not the case although CPAP does improve my sleep quality. My main complaints have been poor sleep quality, fatigue and I get brain fog as well. In follow up in clinic sleep studies including one with CPAP I was diagnosed with "no obvious sleep disorders" although both studies showed elevated spontaneous arousals at 54 and 37 arousals/hr. Doctor says they have no way of knowing what is causing these arousals or if they are happening regularly or just occurred because of sleeping in strange place with equipment hooked up etc. Their only idea is to trial different medications to see if any of them help.
One of my main improvements over the years was with diet changes (removal of dairy, gluten and reduction of carbs/sugar) and I challenge anyone that suffers from IBS, reflux, nasal congestion etc to try elimination diets to try to find any food sensitivities etc that may be causing/worsening not only these more obvious gastrointestinal symptoms but also sleep and general well being.
Other main improvement was with an SNRI antidepressant which helped my energy levels, brain fog, muscle relaxation and more. Interestingly this makes me feel better even at times when it shortens/worsens my sleep.
I also use a sleep aid that helps me get a good night sleep and counteracts the SNRI's shortening of sleep. I think it helps but maybe leaves me feeling a bit drowsy some mornings.
I am finally getting close to feeling good and think that things will continue to improve as I have more time on these medications (only been on the SNRI for a couple months now). Going to be trying one more medication that is used off label for both insomnia and bruxism (as that is one of my issues) and depending on how I feel if I need more improvement will probably see a psychiatrist to see if they have any more ideas.
Too many people poorly understand sleep quality, depression, brain fog and more because they are hugely complicated interwoven processes/symptoms affected by physical, chemical and psychological effects. I just started reading a book called Behave by Robert Sapolsky that discusses neurobiology and how our thoughts, behaviors, life situations, genetics etc all affect our bodies operation and although it is a fairly technical book (has to be for the topic) I think it is going to be very good and a couple chapters in I have already learned a few things.
I mention some of this because PLM for an example can be caused by numerous things including mineral deficiencies (iron is the main one I am aware of) and is often treated by dopaminergic medications (if it needs to be treated). What causes your PLM could also be causing your brain fog and who knows what else. Unfortunately modern medicine does not yet have the technology/knowledge to accurately test, diagnose or treat these issues yet. Sometimes realizing how things like this can all be intertwined and related is helpful in breaking free from the idea that some basic thing like apnea is the root cause of all your issues and helping further diagnose the problem.
One last note again this stuff is all hugely complicated and intertwined. I am not saying you don't have sleep disordered breathing and even if you don't have it I am not saying CPAP doesn't help. Only you can determine that based on how it makes you feel and based on your comments it has helped in your case by helping you get some nights of good sleep and reducing PLM etc. Even if your sleep disordered breathing is minimal like my own CPAP can still be helpful and you determine this by how it makes you feel, not the AHI number.