Why would my UPS cycle on/off at about the same time each night?

[RKP5637]

I have an APC Smart-UPS 700. At about the same time each night it goes from standby into battery mode. The lights aren't blinking in the house, nothing, but it starts going into battery mode about every 5 seconds. This goes on for about 15 minutes or so intermittently and then it stops. It's like the power company is doing something?

Supposedly this UPS also monitors the AC frequency and will go into backup mode if it detects the frequency wandering. I've checked the AC line voltage, it's stable at about 125 VAC. I've heard that sometimes cycles are lost during the day because of loads on the grid, etc., and sometimes power companies increase the frequency at night for awhile to make the same number of cycles delivered each day constant. Otherwise electric clocks, for example, would lose time. Just rambling here, but I've decided there is nothing wrong with my UPS, I guess.