What you need to be mindful of, is the support that the daybed provides.
There are 3 types of support...
1) Link spring with no center support
2) Link spring with a center support.
3) Rails and Bunk Board.
All day beds can accommodate any of these types of support.
Of these, the third provides the best support....so get that if you can.
The first is something you do not want. Any mattress, on this link spring, will sag before too long.
Otherwise, any mattress works with a daybed.
For sleeping, the Simmons Beautyrest Great Lakes Cove in Luxury Firm, is an excellent choice....having an individually pocketed coil system, which contours to the shape of the body and maintaining the natural alignment of the spine.
FREE Shipping on all Simmons Beauryrest Recharge Classic Mattresses From The Mattress Expert. These Mattresses are built with support and comfort in mind!
Another caveat is in how the daybed is used...and by who.
Daybeds are also used for sitting.
The longevity of the mattress will depend on the size of the person sitting on it.
The Simmons Beautyrest is not terribly strong and will wear more quickly when sat on....concentrating all the body weight in one spot.
The coils in the Simmons Beautyrest is made of raw steel...not tempered...and, the upholstery is weak.
If it will be used for sitting, on a regular basis, then the best choice changes.
These Two-Sided mattresses, also use a pocketed coil. The steel IS tempered and more resilient, and there's no memory foam.
Of them, I suggest the Brolynn Plush model for comfort.
Old Fashioned made Two Sided Mattress that you can flip. Strong coil unit and NO Memory Foam. Durable for Plus Sized People. Works with Adjustable Bed Bases.
Delivery is free and there's no sales tax.