Hi there -- Thanks for your question. Technically speaking, any twin-size mattress can be used on a day bed, but some will be better than others as both a sleeping and a couch surface.

When being used for sleeping, many factors will determine its suitability, as described in detail in our review of the Puffy mattress -- as well as in our quiz, editorial guides, and other in-depth mattress reviews.

When being used as a couch, all else being equal, softer is generally going to feel a bit better. A typical couch would be rated as a "Soft" on our softness scale (if it were a mattress). This makes couches too soft as a regular sleep surface for most people, but very comfortable for lounging when your body's weight is concentrated on just your posterior. As such, for day beds we might recommend something in the Soft realm or a bit firmer -- eg, Medium-Soft or Medium -- depending on the needs of the person who would be using it as a mattress, and how often it would be used as a mattress. We would also recommend little to no memory feel (this would be an unfamiliar feeling on a couch) and no more than average cushioning depth. A little bit of bounce might be nice, but is more of an optional thing.

Getting back to the Puffy, as a couch, this mattress is a Medium on our softness scale, so as a couch goes it will be a bit on the firm side, but as a sleeping surface it will be a bit more suitable. It has average cushioning depth, which is as deep as you'd want to go, a little bit of memory feel which is not ideal but not enough to be problematic, and no bounce. Net, while it would certainly be far from my top choice as a couch surface, it is workable. In terms of its suitability as a mattress, you can check out our in-depth review of it for an assessment of which types of sleepers would be best-served by it:

https://www.goodbed.com/reviews/puffy-mattress-review/

I hope that's helpful.