This is a known "feature" of some access points (AP), especially those used on big corporate WAN's. They want to keep the hardware costs low and software development/testing costs low, so instead of making sure the AP is absolutely solid in operation, they reboot it once a day to clear any problems. Even if the AP has dropped off the network it reboots at a preprogrammed time and gets back on the network. Also saves sending someone out to press a reset button. It also gets rid of any attack code that may have taken over via RAM but which is not present in the firmware PROM. I worked on a campus where all our wireless AP's would reboot every night at around 3am. Data collection for experiments in labs would either have to have retries built in, or use the wired network. This is not an uncommon commercial tactic because it takes an enormous amount of testing to try every possible situation a router or AP could get into (malformed packets, unknown bugs in standardized protocols, compiler bugs, overheating when a sunbeam shines on the router, ...) You might be able to change the time this reboot happens in the admin settings of your router to be something more convenient for yourself. That could also ensure you that as to the cause of the restart.