The key factors for a mattress are comfort and support.
The correct support will depend partly on your weight, profile and sleeping position. To keep your spine properly aligned, the mattress should neither be too soft nor too hard. Too soft and it will not support your hips and spine; too hard and it will cause pressure on your hips and shoulders. A general rule is that the greater your weight, the firmer the mattress that is needed to support you. We'd advise you to avoid a very firm mattress if you sleep on your side and have wide hips.
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