For people who live in quite small apartments, there might not always be space for a guest bed or even a sofa bed.
Bunk beds are probably the most space efficient type of bed. They are more space efficient than most sofa beds that require space to fold or pull out, however if only one person will be sleeping in it most of the time then the extra bed is wasted. Instead, if it was possible for a double bed to convert to a bunk bed when required, then the main user will have a more comfortable sleeping space most of the time and also have a spare bed when there is a guest.
This can be done by having half the bed on struts that are hinged. The user will just have to lift one side of the bed and it will lift up in an arched path to a position above the other half. Once there, it will have to click or lock in place to prevent it falling back. A ladder can also be built into one side of the bed that can then be extended down.
The lifting mechanism will probably have to be similar to a car boot lid, in order to help ease the weight for the user. Having the bed lift up this way could also mean that if the locking mechism was to fail then the upper bunk would less likely land on the bed below.
As always, if you think you have a solution or if you have a problem you would like me to try to solve then please feel free to comment it.