Why is there no article "the" before "bed" in "in bed"?

Question

Jamaica Inn episode 5 from BBC learning English drama section:

The dialogue says:

Mary: My aunt is in bed.

She doesn't say -

My aunt is in the bed.

That section is for teaching English. Are they using incorrect English?

Answer 1

It is correct (read "grammatical"). Though bed is normally used as a countable noun, it's better to think of it in in bed as uncountable.

This is common in common fixed expressions about place, time, and movement.

Here are examples of such expressions (according to Practical English Usage by Michael Swan, 70.1 common expressions without articles):

to/at/in/from school/university/college
to/at/in/into/from church
to/in/into/out of bed/prison
to/in/into/out of hospital (BrE)
to/at/from work
to/at sea
to/in/from town
at/from home
leave home
leave/start/enter school/university/college
by day
at night
by car/bus/bicycle/plane/train/tube/boat
on foot
by radio/phone/letter/mail

Also note that even though these fixed expressions are commonly used without any article, you can use an article if it's appropriate in your context.

In other words, you can use such a noun with a/an or the, or even in the plural (e.g. I've lived in several towns), but keep in mind that it will convey a subtly different meaning.

For example,

• Have you tried the new bed yet?
• (a department store trainee asking the manager) What should I do if a customer wants to sleep in a bed?
• (an example of such fixed expressions) "Where's your sister?" "She's still in bed." "Wake her up!"

The main factor for choosing the right article or omitting the article is, as always: context!

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Bonus: Some dictionaries that keep English language learners in mind may make it a little clearer by defining these nouns in common fixed expressions as both "countable" and "uncountable". For example, here is the first definition of bed given by Macmillan Dictionary:

bed
[countable/uncountable] a piece of furniture that you sleep on, consisting of a soft comfortable part called a mattress and a base
double/twin/single bed: The room had two single beds in it.
out of/in bed: It’s midnight – why aren’t you in bed?
get out of bed (=get up): I never get out of bed before 10 a.m.
go to bed: I’ll get home at 11 p.m. and go straight to bed.
put someone to bed: Sam was upstairs putting the children to bed.
make the/my/your etc. bed (=make its covers straight after you have slept in it): Why can’t you kids make your own beds?

You can use these dictionaries to verify the noun in a fixed expression you may find while reading or listening to your materials.

Answer 2

I find the idea to consider bed uncountable really funny, even if some dictionaries say so. English has a strong tendency to drop the definite article when it achieves next to nothing. The aunt sleeps in the bed she sleeps every day in, so why say "in the bed" when the shorter "in bed" says exactly the same. By omitting the article "bed" doesn't become uncountable. I would say such theories taken from the air confuse learners still more. They must think English is a queer language.

The same phenomenon can be seen in German. Instead of "in dem Bett" (in the bed) German contracts "in dem" to "im" and says "im Bett" (in bed) to get a shorter expression.