elh,
You might want to check out some articles about underachievement in gifted children. Here is a link to the topic on the Hoagies site. http://www.hoagiesgifted.org/underachievement.htm

I have younger children, so I can't comment from personal experience. I have heard others say that some of the emotional fallout from being underchallenged really starts in the preteen years and can manifest as underachievement.

That said, I would definitely talk to your pediatrician about the change in sleep, appetite, and mood. You want to make sure nothing physical is going on and/or whether a referral to a specialist is necessary.