Photograph: Julia Forbes

Like any Sleep Number smart bed, the ComfortNext Lux offers 100 firmness levels, and the one you select is your “Sleep Number,” or the firmness you prefer to sleep on. I usually skew toward firmer for lumbar support and weight distribution, which has previously been in the 40 to 50 range. Then, a new recommendation popped up in the app—my firmness level was in the mid-70s, but going softer could improve my sleep score, which Sleep Number calls its “SleepIQ score” and reports in its app. I decided to lower it to a much softer firmness level of 35.

The SleepIQ score summarizes your nightly sleep metrics, including heart rate variability, breathing rate, and time spent in each sleep stage. This data is compiled into a score that summarizes your sleep quality. The app’s score always skewed lower than what my Apple Watch SE would track. However, with dogs jumping in and out of bed, the sensors track that motion and take it into account, too. Despite this, my Apple Watch showed a six-day streak of high-80s to 90s sleep scores. Given that my body is always getting used to new beds with a revolving door of mattresses to test, this is extremely rare for me. Previously, while testing other smart beds or standard mattress types like memory foam or hybrid, I’d achieve this kind of quality sleep with medium-firm or true firm mattresses, never soft ones.

What was clear after two weeks of testing was that, even at the absolute firmest 100 setting, the ComfortNext Lux still felt only medium-firm. I started off with firm and went way softer, but the reality is that the ComfortNext Lux’s plush foam layers will always make whatever level you choose feel softer. If you are seeking a truly firm mattress—like one of our favorite firm options, the Plank Firm Luxe—the ComfortNext Lux does not even remotely play in the same league. This is not shocking, as nearly all smart beds I’ve tried lack the rigidity you’d find in coil-based designs.

Got Your Back

If you regularly deal with back pain or have more severe spinal conditions such as scoliosis, I don’t recommend this smart bed for you. My husband is part of this club, and according to him, the ComfortNext Lux is doable for a few days. After a bit, he needs to return to a firmer mattress to get the support his spine needs. To this end, the ComfortNext Lux’s lumbar support is not where it needs to be for full-time stomach sleeping or heavier-bodied sleepers who require lift around their midsections. Some adjustable bases will have lumbar support bars that project from the base, which would be a beneficial remedy here. Unfortunately, this feature isn’t available on Sleep Number bases at this time, and I don’t recommend pairing this smart bed with an adjustable base from another brand.

If not for back pain, you may wonder who the ComfortNext Lux works best for. In short? Side sleepers. As a side sleeper myself, this mattress offers the level of cushioning I often crave for my pressure points, with a satisfying amount of sink-in that makes the joints feel cushioned. You truly feel cocooned, and not in a claustrophobic way. For couples who happen to be side or side-back sleepers, the foam’s prominence in the ComfortNext Lux also helped prevent motion from rocking the bed. However, it is a bit harder to move between sleeping positions because you are working against the softer surface as you roll.