People with a bad back should not shell out for a hard mattress, say doctors.
A good mattress could ease back pain
A trial published in The Lancet medical journal suggests a medium-firm mattress is the best for a painful spine.
The Spanish study found more than 300 patients to test beds, and found those on the softer mattress reported less back pain in the morning.
Back pain is one of the most common complaints reported to British GPs - it is estimated that up to 40% of adults have suffered from it at some point.
Some studies suggest the problem costs the UK economy billions each year, with 11 million sick days blamed on it.
The research, carried out by doctors from Madrid, Majorca and Barcelona, focused on "non-specific" back pain which is not directly linked to degenerative diseases such as arthritis, fractures or injury to the spine.
New beds for old
Each of the hundreds of patients on the trial had their mattress at home swapped for a new one costing hundreds of euros apiece - half were rated "firm", and half "medium-firm".
After three months in place, patients were asked for information about the pain they experienced while lying in bed, getting up in the morning and overall disability.
On all three counts the medium-firm mattress fared better.
The patients were nearly two-and-a-half times more likely to report improvements in pain in bed, almost doubly likely to say their pain on rising had got better, and more than twice as likely to report improvements in their general disability levels.
The researchers wrote: "The medium-firm mattress was associated with an improvement in disability related to low back pain.
"This effect is particularly important since, although some interventions slightly improve the level of disability, very few medical or physical interventions have achieved this objective."
If patients reported their pain had actually worsened during the trial, the research team bought them another one "for ethical reasons".
Jenny McConnell, from the Centre for Sports Medicine Research and Education at the University of Melbourne, Australia, said the results were important.
"Clinicians are often asked by patients with low back pain about the effect of the mattress on their symptoms and whether a different, perhaps firmer mattress would improve their symptoms.
"Until now, there has been little empirical evidence to support a clinician's recommendations. There is a belief in the orthopaedic community that a firmer mattress offers better support for patients with low back pain.
"However, if a mattress of medium firmness is now improving rather than exacerbating symptoms, physical treatments might prove more effective in future."