you are ill,you had better stay ____ for sone day横线上填in bed

you are ill,you had better stay ____ for sone day横线上填in bed 还是in the bed?为什么?
主要是为什么?

in bed.
in bed 有卧床不起,生病休养的意思.
in the bed 就是在床上,并没有特别的意义.