by Most memory foam mattresses reach 90% expansion within 4 to 6 hours of unboxing. Full expansion to the stated dimensions takes 24 to 72 hours, depending on foam density, mattress thickness, and room temperature. The Amerisleep AS3 uses Bio-Pur open-cell foam that decompresses faster than standard memory foam and arrives with a 100-night trial if the feel isn't right.
Amerisleep AS3
9.1/10
- Open-cell Bio-Pur foam decompresses faster and sleeps cooler than standard memory foam
- HIVE 5-zone support layer reinforces the lumbar zone specifically
- CertiPUR-US certified, made in the USA, ships compressed in a box
- 20-year warranty (years 1-10 free replacement; years 11-20 at 50% off)
- Softer edges than a coil hybrid
- Heavier sleepers over 230 lb may prefer the AS5 Hybrid for extra support
The AS3 hits the medium-firm sweet spot most sleepers need, with Bio-Pur open-cell foam that decompresses cleanly on a flat surface. It's our top pick for anyone shopping for a memory foam mattress-in-a-box.
How long does a memory foam mattress take to expand?
A memory foam mattress is made of viscoelastic material that is compressed, rolled, and sealed during shipping. Once you cut the plastic wrap and unroll it on a flat surface, the foam begins pulling air back into its open-cell structure. Here is the realistic timeline for most brands:
- First 4 to 6 hours: The mattress reaches roughly 80 to 90 percent of its full height. It looks like a usable mattress and you can lie on it, but the foam has not fully recovered.
- 24 hours: Most manufacturers say the mattress is ready to sleep on. By this point the foam is at 95 percent or more of its stated dimensions.
- 48 to 72 hours: Full expansion, especially for thicker mattresses over 12 inches and denser foams. The corners and edges are usually the last areas to reach full thickness.
Room temperature affects the pace. Memory foam expands fastest between 68 and 72 degrees Fahrenheit. Below 60 degrees the foam stiffens and may take the full 72 hours or longer. If your mattress arrived during cold weather, let it warm to room temperature before cutting the packaging.
Three factors that affect expansion time
Not every memory foam mattress decompresses at the same rate. Three variables drive most of the difference:
- Temperature: Heat reduces the viscosity of viscoelastic foam, making it more fluid and faster to expand. A warm room at 70 to 75 degrees Fahrenheit cuts hours off the process compared to a cold room below 65 degrees.
- Density: High-density foams above 5 lb per cubic foot take longer to decompress than standard 3 to 4 lb foams. Premium brands using denser memory foam often recommend the full 72-hour wait. The Amerisleep AS3 uses Bio-Pur open-cell foam which decompresses faster than traditional closed-cell high-density memory foam.
- Thickness: A 12-inch mattress has more material to decompress than an 8-inch mattress. Pillows take the longest per inch because they are dense and compact. Thin toppers expand the fastest.
What happens if you sleep on it before 24 hours?
Most manufacturers say sleeping on the mattress before it fully expands is fine. Your body heat and weight actually help the foam decompress. The mattress may feel slightly firmer than its final form during the first night, and the corners may not be at full height. None of this causes permanent damage.
The one reason to wait is defect detection. Some flaws, including uneven expansion, low spots, and manufacturing splits, only show once the foam has fully recovered. Waiting the full 24 hours before making a judgment call on fit and feel is worth it for that reason alone.
The initial hours after unboxing also include an off-gassing period, when the foam releases volatile organic compounds stored during compression. The smell is normal and typically clears in 3 to 7 days with ventilation. CertiPUR-US certified foams like the Amerisleep AS3 have lower VOC emissions than non-certified alternatives.
How to make a memory foam mattress expand faster
Four steps consistently speed up the process:
- Unbox it immediately. The longer it stays compressed, the longer expansion takes. Cut the packaging as soon as it arrives.
- Remove all packaging completely. Take off the plastic wrap entirely. Do not just cut a slit. The foam needs airflow on all sides to decompress uniformly.
- Warm the room. Set the thermostat to 70 to 72 degrees Fahrenheit before unboxing. A space heater placed a few feet away works if your HVAC cannot reach that temperature quickly. Do not apply direct heat to the foam surface.
- Walk on it gently. Light pressure from your knees, working from the center outward, helps break up compressed foam cells. Some people roll a rolling pin across the surface.
How long does a gel memory foam mattress take to expand?
Gel memory foam takes the same 24 to 72 hours as standard memory foam. The difference is that gel particles are infused into the foam structure to help dissipate body heat, not to change the mechanical properties that govern expansion. A gel foam with higher density will still take longer than a lower-density gel foam, just as with standard memory foam.
The gel infusion does not add meaningfully to compression time in either direction. If your gel foam is not expanding after 72 hours in a warm room, that is a manufacturing defect, not a characteristic of gel foam.
Toppers and pillows: how long do they take?
Memory foam toppers expand faster than full mattresses because they are thinner, usually 2 to 4 inches. Most toppers reach full height within 24 to 48 hours at room temperature.
Memory foam pillows take the longest relative to their size. A dense standard pillow can take 48 to 72 hours to recover fully because the foam is heavily compressed into a small profile. Shredded memory foam pillows expand almost immediately since each loose piece has minimal compression to overcome.
What if your memory foam mattress will not expand?
Wait the full 72 hours in a room above 68 degrees Fahrenheit. If the mattress is still noticeably below its stated height after that, take photos showing the gap and contact the manufacturer. Under-expansion after 72 hours is a manufacturing defect covered by most warranties and sleep trial policies.
Signs it is a genuine defect versus normal slow expansion: the mattress has a permanent low spot in one area rather than being uniformly under-expanded, or the height difference is greater than 1 inch after 72 hours in a warm room.
Saatva Classic
8.8/10
- Ships fully assembled via white-glove delivery, zero expansion wait
- Dual-coil construction with a zoned lumbar reinforcement pad
- 365-night trial, lifetime warranty, free old-mattress removal
- Heavier and not bed-in-a-box; requires white-glove scheduling
- $99 return fee applies during the trial period
If you want to skip the expansion wait entirely, the Saatva Classic arrives fully built via white-glove delivery. It is not a memory foam bed but it is the strongest coil hybrid alternative with a longer trial than most.
Related guides
- Best memory foam mattresses 2026: our full tested roundup with firmness guidance and current pricing
- Best mattress for back pain: zoned support picks for lumbar and mid-back sleepers
- Hybrid vs memory foam mattress: which construction fits your sleep style
- Best cooling memory foam mattress: top picks for hot sleepers who still want foam feel
Give your memory foam mattress at least 24 hours to fully expand before making a verdict on feel. The Amerisleep AS3 uses open-cell Bio-Pur foam that decompresses faster than dense traditional memory foam and comes with a 100-night trial if the final feel is not right.
Frequently asked questions
Can I sleep on my memory foam mattress before it fully expands?
Yes. Most manufacturers say sleeping on it within hours of unboxing is fine and your body heat helps it decompress. The mattress may feel slightly firmer than its final form, and corners may not be at full height. It will not damage the foam.
How long does a memory foam mattress take to fully expand?
Most memory foam mattresses reach 90 percent expansion within 4 to 6 hours. Full expansion to the stated dimensions typically takes 24 to 72 hours. Thicker mattresses and cooler rooms may require the full 72 hours.
What if my memory foam mattress does not fully expand?
Wait the full 72 hours in a warm room above 68 degrees Fahrenheit. Walk on the mattress gently to help break up compressed foam cells. If it still has not expanded after 72 hours, contact the manufacturer. Under-expansion is a manufacturing defect covered by most warranties.
Does temperature affect memory foam expansion?
Yes. Memory foam expands faster in warmer rooms. If your room is below 65 degrees Fahrenheit, the foam stiffens and slows considerably. A room at 70 to 72 degrees cuts several hours off the expansion time compared to a cold room.
How long do memory foam toppers and pillows take to expand?
Memory foam toppers typically reach full height within 24 to 48 hours. Pillows take 48 to 72 hours because they are dense and heavily compressed. Shredded memory foam pillows expand nearly immediately.
Amerisleep AS3
9.1/10
Medium-firm Bio-Pur open-cell foam, HIVE 5-zone lumbar support, 100-night trial and a 20-year warranty. Our top memory foam mattress pick for 2026.
This guide is part of our Mattress Buying Guide hub , compare all the top picks and narrow down your choice there.