on the bed 没有睡着,或者在床上但是没有躺下
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.（因为英英字典的解释是：after a very lengthy period of time）
in bed尤指在行房过程中或者在床上做事的过程中.比如：He performs well in bed.又如：Jack and Jane had breakfast in bed.