https://www.quora.com/Which-is-correct-in-bed-or-on-bed
The phrase “on bed” is not used in English. We can say “on the bed” or “
on a bed”—but we never say “on bed” (with no article). “On a bed” or “
on the bed” refers to a place or a location. Here are some examples of how
we use “on a/the bed”:
She sat on the bed and put on her shoes.
He put his suitcase on the bed and opened it.
Most little children love jumping up and down on a bed.
Notice that in all these sentences, the phrase “on a/the bed” tells us
about the place where the action in the verb happens. The action of sitting
happened on the bed. The action of putting happened on the bed. The action of
jumping happens on a bed.
Keep this idea of place or location in mind as we look at the phrase “in bed
” next.
The phrase “in bed” (without any article) is an idiomatic expression that
we used to say that a person is sleeping as a regular nightly activity, or
that a person is lying down in a bed because he or she is ill and needs to
rest or is too weak or frail to walk around.
So, the phrase “in bed” refers to a state, not to a place or location. It
is similar to expressions such as:
in school (→ the state of receiving formal education)
in hospital (→ the state of receiving medical care in a special place built
for that purpose)
in church (→ the state of attending a religious service)
in jail (→ the state of being locked up as a punishment)
in prison (→ the state of being locked up as a punishment)
in court (→ the state of attending a legal proceeding in the presence of a
judge)
in debt (→ the state of owing money or something similar to someone)
Notice that all of these expressions do not use any article with the noun.
This is because the noun is treated as a non-count (uncountable) noun, and
the whole phrase refers to a state. If we want to refer to a location, we use
an article with the noun. The following sentences illustrate the difference
between state (wihtout an article) and location (with an article):
My grandmother is in hospital with a broken hip. (state of receiving medical
care or treatment)
→ Right now, my parents are in the hospital visiting my grandmother.
(location)
We do a lot of singing in church. (state of attending a religious service)
→ The tourists will be in the church for the next 20 minutes. (location)
His uncle is in prison for stealing money from the company that he worked
for. (state of being locked up as a punishment)
→ My cousin works in a prison as a prison guard. (location)
To summarise this answer, “on bed” does not exist in the English language.
If we want to talk about the place or location where something is, or where
some action happens, we use “on a/the bed”. If we want to talk about the
state of being asleep as a regular nightly activity or a state of being
forced to remain lying down in a bed because of illness or something similar,
then we use “in bed”.