Server storage filling up overnight then decreasing?

I have been having a strange issue and I can't narrow down what's causing it.

Currently if I check my server storage, it's around 60-65% of 120GB but overnight, around the same time (2-3am), I get a "Disk usage warning" indicating that I am over 80% full. This was alarming the first time it happened, but I realized that when I check my storage in the morning at work, it is always 15-20% lower than what it says.

I receive this warning now every night around 2-3am, and it always warns me I am going over, so my storage is going up and down overnight I know the percentage I am checking in the morning isn't just wrong because I receive the warning every night around the same time, indicating I am going over and under the 80% notification threshold every night.

What could be causing this? Is it my provider? They're of little help. How can I see everything WHM/cPanel is doing overnight, to determine if something is updating or something, or something is happening at a set time to increase my disk space, then decrease it?

Thanks.

 

Thanks for the link. I think that might be it, I was just concerned that it was going up sometimes 20% overnight then going back down. It must be to do with the back-up system.

I'll just make sure to keep an eye on when it gets close to 90-95% overnight.