If you accidentally sit on your iPhone 5s would it break?

As most people's beds are at least somewhat soft, even sitting on it would probably be unlikely to do any damage unless there was another hard object involved (you had keys in your pocket, the phone was on top of the remote).

As SergZak suggests, if the phone is working fine, it probably is fine. No need to borrow trouble.

We offer support, not reassurance.

To put it bluntly, the most affecting factor would be your weight.

Seriously though, does it still work? If so, don't worry too much about it. If it starts doing odd things, something may have happened.

SergZak wrote:

To put it bluntly, the most affecting factor would be your weight.

There are other factors, how large an area is that weight distributed across, does enough of that area extend beyond the phone so as to alleviate some of the strain on the phone itself, how rigid is this area, is it insubstantial and floppy or firm and unyielding.

Not sure an answer can be obtained in this manner.