It's been several days. This time I stay up to check exactly when it goes wrong.
The slowing down of internet speed started around 3 AM, when I was playing the online game and I noticed I couldn't pair up with my team. Then I tried rebooting the router several times during the next hour, the internet came back and off. Can't view videos online; can't receive iMessage; Verizon official site couldn't be reached neither on phones or PCs.
It went completely down at 4:41 AM, waited until 5:40, things went normal again. I was so worried as I have a online meeting this morning.
I searched similar events of Verizon Fios, it seems it happens a lot frequently. I just need a notice (of maintenance or upgrading, etc.) in advance, is it very hard for Verizon?