I had it - it knocked me out for a couple of months. It started with tonsillitis and i had a couple of days off school, then it was half term and i just slept for the week, and then i went straight back...i wasn't diagnosed until the tonsillitis had gone and therefore i thought i was better, and in a way i think that helped - if i'd known i'd have been all "oh no i'm ill!" and it would have been more psychosomatic.
Within 3 months i felt fine again, but in that time i was sleeping for a couple of hours every afternoon, and going to bed early. but doing all normal things! However, when it came to revising for exams a couple of months later, i realised i had been a lot more out of it than i realised at the time - i had the notes but no recollection of the lessons from the period when i was really ill!
My advice is just sleep when you want and take it easy, but try and keep going and not get disheartened. The worst bit is some days you feel better and do more, but then the next day you feel even worse again...so sleep more that day and keep you chin up.
I do sympathise, it's horrible. hope you feel better soon!