A spring mattress for beds is disclosed. In at least one embodiment, the spring mattress includes a plurality of strings which are interconnected side by side and include a continuous casing material with a plurality of separate pockets with coil springs enclosed therein. Moreover the casing material for at least some pockets is at least substantially airtight, thus providing a resistance to air being pressed out when the springs of the mattress are loaded, which, when subjected to a uniform load during a transition period, results in a gradually increasing depression of the spring to its depressed state. This results in a comfortable mattress, into which the user sinks gradually and which thus is comfortable and safe and allows the user to move easily. A corresponding method and device for manufacturing such a mattress are also described.