How does it feel when waking up with untreated sleep apnea?

Go get a sleep study. Go get a sleep study. Please. go get a sleep study.

I hear you loud and clear. But is it possible for you to answer the question?

You post a lot of questions about sleep apnea and if your concerns are really so numerous, you should be seen for possible sleep apnea. Many people have told you this AND answered your questions.

Waking up with untreated sleep apnea feels absolutely horrendous.

For me, I felt terrible all the time.
I would wake up feeling as though I'd had five minutes of sleep.
Or, I would wake up because I'd stopped breathing. My heart was hammering, I was sweating, gasping for breath. I thought my heart would explode.
In fact, feeling that way drove me to the emergency room one day. There, doctor told me I almost certainly had "sleep apnea". I'd never heard of it!
Then I got tested, got a CPAP, and got my life back.

Like drained? Like you don't want to get out of bed?

You must have had very severe sleep apnea. Did you snore?

After my sleep study, I was told I was about the worst case the doctor had ever seen. I wasn't really asleep, but unconscious from hypoxia.
Yes, I snored. Like a freight train.

What was your waking life like? Could you make it through the day doing normal things without taking a nap?

I don't really remember. It was more than 25 years ago.
I do recall that I was just dragging along, barely able to function at work.
It had probably been getting slowly worse for years, then finally got to a critical point.

Many people report that they don't feel vastly better after what appears to be successful treatment for their apnea, and so they give up on whichever treatment they have undertaken, including PAP therapy. Some report that they immediately felt 100% better after Night The First, and others go weeks and say they're ready to give up, even though their reported AHI is less than 5 events per hour of use (note that I said 'of use', not 'of sleep' because the machine hasn't a clue if you are awake or asleep...it just monitors its own flow and events that it senses).
Some have modest to excellent success in the way of treatment for their apnea, but by now they're so sleep-disordered that they still feel like the brown goo. They need another type of assistance, but also to continue their PAP treatment.
Some say they do so much better with an AHI of about 3-5 events per hour, and they're willing to continue to use their machines. Some do poorly unless some kind soul coaches them to get their AHI under 3, in which case they finally begin to sleep better and to do better during the day.
Those who are untreated for whatever reason report headaches, brain fog, narcolepsy, cognitive disfunction, memory loss, heart arrhythmia, indigestion, metabolic syndrome, and on and on and on....