Why kumbhakarna sleep for 6 months?

He did a big tapasya for Lord Brahma to get a boon. He wanted to ask for Indrasan (throne of Indra), but all gods got very scared. They prayed to Goddess Saraswati for help. She sat on his tongue, so by mistake, he asked for Nidrasan (a bed for sleeping). Lord Brahma gave the boon. So he was cursed to sleep. Ravana requested to change it, so Brahma said he will sleep for six months and wake for one day.