It may be something to do with "bed" being treated as an abstraction, a bit like "church", "hospital", and "school". As these are the 4 main ones to worry about, you could also just regard the lack of "the" as idiomatic and learn them separately. Linguists may have a better answer. www.englishpage.com has a good articles tutorial.

Basically: Singular indefinite - A/an + noun Singulate definite - The + noun Plural/Uncountable indefinite - (no article) + noun Plural/Uncountable definitite - The + noun You don't put articles before proper nouns. This makes sense, since proper nouns are jjst labels. Saying "a Russia" or "the Russia" implies that you can have "two Russias". You could say "a Korea" because there ARE two Koreas: North and South. This is the general rule, but unfortunately there are lots of exceptions in idiomatic expressions! "Go to bed" is one of them. You can think of "go to bed" as a single expression. "Go to a bed" and"go to the bed" would just mean that you're walking to a bed. "To go to bed" acts as a single verb that describes an action (ie. "To get into bed and fall asleep).