This is also known to happen on N810 and N800. The problems are most definitely memory leaks in one or several widgets. The problem is not apparent immediately after the reboot, but appears several days later. In my case these leaks even lead to hildon crashing and restarting. Extensive testing should be done in order to find the exact root of the problem. This unfortunately is not an easy, nor a fast task. Look around the forum, and you'll find a lot of threads with these types of issues.