Innkeepers around Hyrule offer the option of a 'Soft Mattress' at double the price. When asked the difference though, they reveal themselves to be singularly untalented salespeople, rambling about the bed being stuffed with 'premium cucco feathers' that will leave Link 'more energized'.
Money is tight, so I haven't bothered to try it out. What good is a Soft Bed?
Best Answer
A soft bed grants you an extra heart temporarily.
The IGN Wiki for the Highland Stable states:
This GameFAQs forum says the same, and also states that in Zora's domain, there is a water bed that grants 3 hearts, and a full extra stamina bar: