Fortigate goes to "sleep" until someone goes to the login page and then it wakes up again!

Hello

We are having a really strange issue with our Fortigate 600c. The unit will go to "sleep", usually during the night sometime but the past week it has happened a few times a day, and the campus will loose internet access. As soon as somebody tries to access the unit, you don't even seem to have to log in, it will "wake" back up and usually works for a few hours before having the same issue. Its really odd, I don't see anything in the logs and since the unit starts working as soon as you web in, it makes it very hard to troubleshoot because it is always working when we are logged into it.

Cheers!