An modern inflatable, waterproof matress.
The 'waterproof' aspect to these items is convenient when your tent falls down in the middle of the night and the rain starts to pour. Although your garb and blankets are saturated, and your armour has gone rusty, at least your _bed_ isn't soaked through too!
Medieval equivalents to these items (none of which are very waterproof) include:
- folding camp beds
- straw filled matresses
- mobile rope beds
- wrapped in a cloak on the march