Having seen this myself a number of times (and sick of having to look for the answer), there is an error that occurs in the User Profile Sync - Event ID 6398 - in the Event Viewer it looks like this:
Notice that the error is every minute! This will certainly fill up the logs!
The issue is quite simple - there are many fixes but turns out the problem in simply the Forefront Identity Manager Service is not running (or could be the Sync). Open Administration > Services (or use Run and type in services.msc) and locate the Forefront Identity Services:
Simply start the service(s) (right click and select Start). I am not sure what causes it to stop periodically but in this state, User Profile sync errors will continue frequently,.
Hat tip to this post: