Either it's based on range limitations (of course this is assuming either you use the device somewhere else at 12 am, or something else happens at 12 that causes the range to be further limited) -- keep in mind 5 GHz will not send the signal as far as 2.4 GHz, or, it could be a router setting that's been set up to have the 5GHz stop broadcasting at 12 am (this can be tested by reviewing the settings via https://192.168.0.1/ or by doing a factory reset on the equipment.
For further resolution, do you know if you move directly beside the router at this time of night if it still doesn't show up as a listed network?