Deep sleep is blissful for two reasons. All limitations disappear in deep sleep. You are located in time, space and name and form. The Chandoggyo Upanishad says very beautifully. Yat alpim tat matyam. Tada asu kaam. Deep sleep was never thought so important but it is actually limitlessness. So one reason why it is called blissfulness is the most obvious: There is no samsara there are no problems. Janaka wanted to know this true or he would find no problems if he considered his deep sleep a new way of looking at life.