Jolting awake with sharp inhale, a solution

[McPhearson99]

Executive summary: I'm making this post in case someone else is having this problem and my solution can help them. I was jolting awake with a sharp inhale approximately 15 minutes after falling asleep on an almost daily basis, often multiple times in a row. I figured out by chance that cough suppressant (e.g. NyQuil) completely prevents these wakeups and somewhat improves general sleep quality.

Background: I'm 27M, healthy BMI, no other health problems. I developed upper airway resistance and sleep apnea about a year ago and got a CPAP about 8 months ago. The CPAP greatly improved my sleep quality, letting me function as a person again. However, I was still frequently waking up shortly after falling asleep.

The problem: About 15 minutes after falling asleep, I would wake up with a jolt and a sharp inhale. These "sharp inhale wakeups" could happen in succession: close eyes, fall asleep, sharp inhale wakeup, close eyes, fall asleep, sharp inhale wakeup, etc. The sharp inhale wakeups also happened less frequently around 3AM-4AM but the majority occurred as I was falling asleep. They reduced the amount of sleep I was getting per night, occasionally interrupted my sleep, and in general made me feel exhausted. The sharp inhale wakeups would also happen when I tried taking a nap during the day, even in a sitting position, to the point where I didn't need to set a timer for my nap because I would jolt awake on my own.
I started recording the times when I woke up due to a sharp inhale wakeup by screenshotting my phone's lockscreen. OSCAR didn't show anything unusual other than a spike in flow rate when these happened (see attached screenshots).

Things I tried:

• Raising CPAP pressure: I increased CPAP pressure to a level above what I would encounter on a normal night, to 13cmH2O. On the nights that I used this pressure level, the pressure chart was essentially a flat line. The sharp inhale wakeups still happened.
  
• Changing sleep posture: I tried sleeping on my side and with my head raised. These had no impact.
  
• Advil: On a few nights, I took Advil before going to bed. This was inspired by the fact that Advil helped my sleep quality before I got my CPAP. Taking Advil reduced but didn't eliminate the issue.
  
• Benadryl and Zyrtec: I took Benadryl and Zyrtec on suspicion that allergies might be exacerbating the issue. Benadryl had no effect and Zyrtec caused a small improvement. I suspect this is due to the fact that Zyrtec has a slight anti-inflammatory effect.
  
• Enabling EPR: I enabled EPR on my CPAP and set it to 2 and 3. EPR of 2 improved my sleep and slightly decreased the amount of sharp inhale wakeups. 2 was more helpful than 3.
  

Solution: Due to a cold, I took NyQuil before falling asleep one night and found that it completely prevented any sharp inhale wakeups. I've been taking NyQuil or a generic equivalent for two weeks and I've had zero sharp inhale wakeups, which is unprecedented for me.
NyQuil has acetaminophen, chloropheniramine maleate, and dextromethorphan, where the dextromethorphan is listed as the cough suppressant medicine. The generic cough suppressant I'm using now has dextromethorphan and doxylamine succinate.

Final setup: CPAP pressure range from 7cmH2O to 15cmH2O with an EPR of 2 and I'm taking generic NyQuil about 15-30min before going to bed.

I plan on discussing the issue and medicine with my doctor when I have my next appointment in May to see if he has any insight on why the solution works.