tom'father is ill.he is in bed 还是on bed 或是in the bed抑或是at be

Tom's father is ill.He is in bed.
in bed 生病在床
on the bed 在床上面
in the bed 睡着了,或者是躺在床上,特别是在床上呆了一阵子了
at bed 没有这种用法