Leaving out the defined article with certain idioms

[Guest12-15]

I've always asked myself why e.g. bed is usually  used without "the".  At least it should be used with a possessive pronoun, I think, as it's not really an  institution , such as "church" 

If you were able to help me, I'd be grateful.  

[tiorthan]

It doesn't have to be an institution. The important part is, that it is no concrete definitive object.

"In bed", for example, is not a specific bed but an abstract concept. In fact, it doesn't have to be a bed at all, it could mean a haystack or some other place you use to sleep.

[Guest12-15]

Thanks more than a lot..