
Brooklyn Bedding
Signature Hybrid
Durable, comfortable, adjustable firmness; excellent value.

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Generally, yes. Costco will save you money but will always offer lower quality in terms of manufacturing and longevity. A mattress is one of the most important purchases you can make for the improvement of your every day life - if you can afford it, don’t cheap out on it. The difference in support you get between 800 vs 1200 coils is substantial. There are usually other differences in both warranty, return policies and distributing that can account for the price difference, but coil would be the main factor and for good reason. Go with Sleep Country for this one, their Sertas are specifically made for the company and has exclusivities no other retailer will have.
In short. Your old one is a better quality mattress than your new one. More coils for support, better for quality for there foam. Serta Perfect Sleeper is a better quality mattress at 10 years old vs a brand new Sleepys basic.
For comparison you didn’t just get a Perfect Sleeper, you got a higher end perfect Sleeper that retails for $1300 vs a $500 no brand. Perfect sleeper you had, has over 1700 coils/innersprings. The Sleepy’s Hybrid has 440 coils/inner slings so it has a 1/4 of the support as your previous. So that’s what you’re feeling in terms of quantity. Also I’ll send the foam specs for both too, and it all comes down to quality https://preview.redd.it/e5bf4mhvelpf1.jpeg?width=1179&format=pjpg&auto=webp&s=3774b2a28c255cde9a0d2eac94a77b06040abbc3
But Serta is still higher quality than the Sleepys Basic Hybrid. Especially depending on which level of Serta, perfect sleep collection vs iSeries. Serta makes good products, not top of the line by any means, but I wouldn’t consider them low end. Just about every major brand has a ‘low’ end section to be competitive in price, but that’s just part of the overall picture.
If it’s not in your budget to buy a more expensive mattress with better cooling technology, consider a topper with cooling benefits. Coil for coil you probably had more in the Serta, which is why it may feel more supportive, as the Sleepys basic hybrid is a more basic coil structure/count.
Big Lots was selling somewhat firm Serta Euro twin mattresses for about $80 a couple or so years ago, which was an absolutely awesome deal and a quite high quality mattress. Now, the crazy part is that my fam bought them at Big Lots in Los Angeles and the Bay Area, and I just looked to find out apparently there are now only Big Lots stores in the eastern U.S. It's a real shame, but if you're in a region that still has those stores it's worth a look.
*cracks knuckles* I work at a mattress store and I know more about the specs on these things than any of my customers probably care about. First of all: for your own sanity, do not buy a mattress without trying it out in person first. Even if the online-only purchases come with free returns it is NOT worth the hassle. It's legitimately so much easier to go to a store and have a brief conversation about the way you sleep with someone who knows mattresses. Different mattresses are all made to support different sleep needs, someone who is knowledgeable will likely be able to match you with the best one to address your needs in your budget. With that said, knowing only from your post that you're a side sleeper, here are my luxury hybrid recs. Best of the best: Tempur-pedic. These things are built to last 25 years. Fantastic if you're a hot sleeper, have any pain, and/or toss and turn. (Side note: a lot of people are convinced that tossing and turning is normal. It's not, it's a sign that you're struggling to get comfortable on your bed). Specific model recommendation to start with is the Pro Adapt 2.0 Medium Hybrid. A bed as good as this is an investment in the way you heal, feel, and function, but holy shit is it worth it. I've fallen asleep on this one in the store in five MINUTES by accident while not even tired or intending to fall asleep, just testing it out. I need to physically restrain myself from laying on this bed in between customers. Having had plenty of time to try all the top brands... This one's my personal fave. Great if you don't like the feel of Tempur pedic: Purple. I typically find that people who don't like the feel of Tempur love the feel of purple and vice versa. It's kind of a coin toss. Either way, both fantastically supportive. Purple is also really great for people with chronic conditions; the gel flex grid adapts to support your body no matter what position you're in Very good option if these two are a little out of the budget: Sealy Posturepedic hybrids. Not as durable as the above two, and won't give as much targeted support or pressure relief, but still very comfortable. If you have any hip pain or back pain, posturepedics actually have some extra coils in the middle third of the bed (where your hips/back go) for added support there. FYI, if you're a side sleeper, a firm mattress isn't recommended. A firm puts too much pressure on shoulders/hips for side sleepers. Medium and plush/soft mattresses don't sacrifice support, they just add comfort layers. I'm going to stop here before I never shut the fuck up about my esoteric mattress knowledge 😭😅
Stomach sleepers usually need something on the firmer side so your hips don't sink in too much. I'll give you some "good-better-best" recs that some of my stomach sleeper customers have really liked. Good: Nectar premier memory foam, if you want to keep it around $1000 or under then memorial day sales are gonna be your best friend (many companies go until the end of the month). Even in a medium, nectar is definitely on the firmer side for memory foam because it's denser and higher quality. As far as I've seen, nectar seems to be the most durable for memory foam mattresses. Better: if you don't like the feel of memory foam as much as an innerspring bed, Serta Perfectsleeper with a Pillowtop (I actually have a Serta perfectsleeper! Genuinely very supportive for the price point it's at) Best: if you like the firm feel, Stearns and Foster has an extra firm mattress that's specifically designed for stomach sleepers! Lux Estate 13.5" extra firm. If that feels too hard for your preference, the 15" firm is great too. Edit: OP I'm sorry for being a nerd in your comments section LOL
I have the exact same issue and I’m only a few years older. My problems started right around your age and it’s been an absolute nightmare. The issue can be due to several things and you should take pictures of your back to really see what’s going on with your spine. Most mattresses cause me to sink unevenly with my hips sinking much more than my shoulders. This causes the spinal sagging and leads to lower back and some mid back pain. The only mattresses that I’ve slept well on are the Serta (perfect sleeper specifically) ones with zoned support. My parents have one and I could not for the life of me figure out what it was until recently. I’m not advocating for that brand, it’s the last one I wanted since I’ve had other mattresses from them without the zones and slept horribly. However, I’ve slept on 3 different ones while traveling for work and slept better in the hotel beds than my own. They all happened to be the same mattress which is no surprise given I stayed at the same chain, but it reaffirmed what I believed. A lumbar pillow has helped but it’s nowhere near as good as having the full support. I don’t imagine a soft mattress will fix this if you’re sinking unevenly. Depending on the construction, the weight at your pressure points will cause the mattress get pulled downward further out so you won’t get support at your mid section, you’ll just sink more. Hopefully you’re able to find something that works or at least have more insight on what to research
We recently got a Serta Perfect Sleeper and I absolutely love it.

Brooklyn Bedding
Signature Hybrid
Durable, comfortable, adjustable firmness; excellent value.

Sleep EZ
Organic Latex Mattress
Highly customizable layers, durable; but extremely heavy.

Helix
Midnight Luxe
Pain relief, side sleeper support; but some find too soft.

Purple
The Purple Mattress
Unique grid feel, cool; but heavy, expensive, polarizing.

Brooklyn Bedding
Aurora Luxe Cooling
Excellent cooling, zoned support for heavier sleepers.

Ranked #1
Novaform - Platinum 16.5" Luxury Hybrid Mattress

Ranked #1
Brooklyn Bedding - Titan Plus Luxe

Ranked #1
Purple - The Purple Mattress

Ranked #1
Sleep EZ - Organic Latex Mattress

Ranked #1
Helix - Midnight Luxe

Ranked #1
Sleep EZ - Organic Latex Mattress