Why is waking up when my alarm goes off so difficult and unpleasant yet "sleeping in" doesn't feel good and isn't satisfying?

A complete REM cycle is usually around 90mins, and it takes about 30mins to start falling asleep deeply. For me personally, 7.5hrs is the perfect amount of sleep, which equates to 5 REM cycles.

Now, if you wake up in the middle of a cycle, you will usually feel groggy as your brain gets to grip with reality. Therefore, you want to aim to wake up just at the end of a cycle, ideally.

Try sleeping without an alarm, and make note of when you went to bed, and when you woke up. Hopefully, that time should be roughly divisible by 90mins, assuming nothing woke you up.

Knowing the above info, you can time your sleep better to ensure you don't set an alarm for mid-cycle. As the other commenter mentioned, there are also apps & devices that can help detect when you're coming out of REM and time the alarm based on that.

Lastly, if you do snooze after waking up, try "snoozing" for a full REM cycle (90mins), or try a power nap (<25mins). That might help "catch up" on sleep after you wake up.

I used to purposefully set my alarm 90mins before my actual wake up time, and then "snooze" for 90. That was mainly to induce lucid dreams though.

+1 for sleeping without an alarm. If you really need an alarm to get up every day, you might not be getting enough sleep to begin with. Some of that is unavoidable (noises, light waking you up at night etc.) but a lot of people think they can get by with like 6 hours or less of sleep and just get by on extra coffee.

Taking a week or so to just let your body wake up when it wants (even if it means going to bed early, ugh) will help you figure out how much sleep you need.
Added to that, some smart devices try to measure your exact sleep cycle and wake you at the right time. Phones yrying to do that using microphone or accelerometer to measure sounds or vibrations are notoriously bad at it, some watches with more sophisticated sensors might have better results, but I don't have personal experience with that.
+1 on determining (and understanding) sleep cycles. And +1 on naps. I consider myself really good at napping. Somewhere I read NASA determined 26 minutes being the best nap amount, so I have a 28 minute timer because I can usually fall asleep within 5 minutes.

Routines before sleeping can help your brain fall asleep faster. I beleive that's a key to my nap ability because it usually works even if I don't think I'm sleepy. I have 1 hour for lunch at work, so if I bring food, I can nap in my car. I used to park facing uphill and east at one job to get extra recline and keep the afternoon sun off me, but my current lot is flat and I got a sleep mask. I play a specific artist (Deadmau5) on a low volume. I have used construction ear muffs as extra sound dampening.

I learned of a mental game that does an amazing job of getting my thoughts to calm down. Pick a category, then run through the alphabet to name something for each letter. Apple, banana, cranberry, etc. If you can't think of something, skip the letter and come back. I basically never make it to the end unless I'm really upset about something. And, not gonna lie, the most effective category for me is sex/porn categories. Nothing is more focused than a horny brain.

My most pleasant waking up experiences (in terms of my own body, not who was with me) were when I worked nights, and woke up around 4-4:30 pm.

On a day shift schedule I often feel too warm when I go to bed, and too cold when I wake up. When I worked nights, the cool mornings and the warm afternoons canceled both of those out and felt great.

As such, I only needed 6 hrs sleep when I worked nights. Now I feel groggy unless I get 8.

I set my thermostat schedule to help with that. It doesn't work too well in the summer due to weak ac, but the heat works great in the other 3 seasons. I let it drop to about 65 around 11pm, raise to 72 around 8am, drop to 68 at 9am if I'm home or lower for work days.
- +1, I have our thermostat kick down an extra degree at night to tell our bodies that it's sleeping o'clock. It makes. A huge difference in the quality of sleep I get.
  
  A warm shower or bath followed by cooling tricks your body into going into its sleep cycle and producing melatonin.

Use smart lights. Have the light gradually ramp up intensity until it peaks a few minutes before your alarm goes off.

Also make sure you're going to bed with enough time to fall asleep and get a full night's sleep consistently.

Smart blinds can also serve a similar function. My bedroom catches some light from passing headlights and there's enough light pollution to affect my sleep. However, my SO really struggles to wake up if it's dark. Smart blinds have been a great solution for us. The biggest drawback is noise. Affordable blinds are not silent.

Do you have a sleep cycle alarm clock? It might wake you up a little earlier to make waking easier.

As others have explained, waking up at the wrong point in a sleep cycle feels crappy. But also as the night turns to morning, your sleep cycles change and sleep becomes lighter - waking up is a process, not a moment. So get up when you wake up naturally and you should be good, but also keeping to a schedule helps, so if you are naturally waking up at 7am or whatever, going past that time can interfere with, well, falling awake.

I get you though - so often I feel like sleep is so close by I just want to go back.

My husband handles this with coffee, he gets up, makes coffee, drinks it in bed (while I sleep through all this) and then either feels awake, or, if he does go back to sleep the caffeine helps him wake back up, like a coffee nap. I can't do that, prefer to wake up then coffee a few hours later, but it works for him.

Do you know about the rerack? Wake up early, read a bit or check the news or whatever, then go back to sleep for an hour or two. Best sleep ever! Feels like you're cheating the sleep rations.

Everyone else has good suggestions, but for me, it turned out to be sleep apnea. See if you can talk to a sleep doctor.

Seconded! I found out in my late twenties that I was waking up every three minutes, all night long. I used to sleep 10-18 hours a day if I could get the time, and was still exhausted.

I won't even nap without my breathing machine now; the difference is shocking.
- My nurses always complement me for using it 99% of the time. I can't understand why someone would not use it.
- Late twenties as well. Unfortunately, my machine hasn't made a difference for me yet, which has been pretty difficult. Hopefully a solution is in my future

my sleep routine has become one of if not the most important routine. i dealt with insomnia for years, which always spirals into poor appetite, ever-decreasing functioning and bad decisions. i understand now that maintaining a routine is critical for me, and i have added tools for falling asleep, staying asleep, and waking well-rested.

i picked up a good sleep mask, one that blocks absolutely all light and allows unobstructed blinking. even more so than the functional aspect of blocking light, the sensation of wearing the sleep mask has helped program my brain into quickly recognizing sleeptime on-demand.
a minimally-intrusive sleep aid. i started with melatonin but eventually switched to D8/D9 gummies and never looked back. within 45m of taking a gummy i feel the heaviness in my limbs and easily succumb to sleep. this is likely not necessary for everyone but i consider the reliable effectiveness to be an effective insurance.
ensuring proper sleep posture. i overlooked this one for a long time, thinking that stacking pillows under my head and around my body was as good as it could get. but i would wake up every morning with neck and back pain, grumpy and frustrated. after learning about traditional asian pillows it clicked together that the intent of a head rest is neck support. on a whim i picked up a buckwheat hull neck roll and it has absolutely changed my life. first time i used it, flat on my back with it under my neck, my legs tingled from the relief of tension in my body. the moral here is to assess for sources of discomfort in your sleeping posture and find ways to improve it so you are not wasting time and effort trying to get comfortable.
consider breaking up your sleep hours. I started an unconventional routine last year and it has been very helpful for me in waking refreshed. i set my first alarm for 2.5-3 hours before i want to be fully awake. when that goes off i manage to drag myself out of bed, have a light breakfast and move myself to a recliner in the living room where i gently doze off for another hour/hour and a half, with a few extra minutes on both sides. initially it was not super easy to fall back asleep quickly, but now that it’s routine and with the help of the weighted sleep mask, it happens in minutes. by the time my second alarm goes off, if i’m not already beginning to stir, i open my eyes and am absolutely ready to go. knowing that i can doze off again makes getting up and relocating on that first alarm much much easier, and i have come to deeply enjoy the dedicated peace and quiet of that early morning nap; no expectations weighing on me first thing, just focusing on my breathing and relaxing my body. the downside is now that i’m used to this pattern, my body expects that morning nap and i absolutely have to be up early enough to allow time for it.

i’m certain that a persons ideal sleep routine is extremely subjective and unique to that individual, so these tips may not all be helpful for you, but hopefully they give you some ideas of new strategies to try.

by the time my second alarm goes off, if i’m not already beginning to stir, i open my eyes and am absolutely ready to go. knowing that i can doze off again makes getting up and relocating on that first alarm much much easier, and i have come to deeply enjoy the dedicated peace and quiet of that early morning nap

I also have issues with insomnia (due to bi-polar) and frequently wake in the early morning and relocate to the sofa. I don't know why this is helpful in getting back to sleep, but changing the venue, so to speak, helps me instantly fall back asleep.
- I remember hearing somewhere (probably some podcast) that it used to be normal for people to sleep for 3-4 hours, then wake up and do stuff for an hour or so, then go back to sleep for 3-4 hours again. Maybe to help keep watch while you sleep or something similar?
  
  It's about 50/50 for me. Sometimes getting up and reading for an hour or so will let me fall back to sleep instantly, other times I'm just wide awake at 2AM and then magically get sleepy at 6:30 right before work.
- 100%. i’m rocking AuDHD, that perpetual struggle between need routine and must have novelty lol. interesting to hear that the ‘mid-show venue switch’ is helpful for someone else. feels like a hack somehow.

You been checked for sleep apnea?

I Think i have sleep apnea but i don't want to to pay to greedy US healthcare. Can I self cure this ?
- That is a great question. I'm not a doctor, but I do know there are different kinds of sleep apnea, like your airway isnt collapsing, but because your central nervous system has problems. I started using my father's old CPAP and noticed an improvement, but I also had a previous sleep study with enough hints for me to pursue it further. I know US healthcare isn't ideal, but doctors also know the answers ya dig?

Depression?

It might not help you, but it helped me to recognize when laying in bed trying to "sleep in" wasn't going to happen. If I'm not asleep again in 5 minutes and it's past a certain time (6:30 for me), fuck it. My body just aches if I lay in bed awake trying to sleep. I get up and enjoy the quiet morning.

Maybe you have a vitamin deficiency. You can get tested at your doctor.

Or just buy a multivitamin and have one daily. It is much cheaper than test at doctor.
- Sure, but it is good to know if something is wrong.

If you want to know how to fix this problem, the term to search for is sleep hygiene. You've been waking up at the wrong times in your sleep cycle. Quality sleep is a matter of habits, every person is a bit different. Research and apply what fits best with your lifestyle. Also, invest in quality beds and pillows, as well as temperature regulation. It's far more important than most people give it credit.

This Nedroid comic explains it:

https://nedroid.com/?452

Idk if part of the unpleasantness is due to waking suddenly, but if it is consider different alarms like maybe music, an alarm that gradually gets louder, a quiet alarm, or a vibrating alarm. Some have also mentioned alarms with lights that gradually get brighter.

I grew up with one of those cheap ass, plug in, buzzing alarms with the little peg you push in on the back and I still jump and freak out when I hear that sound.