system memory reset iOS 9.1 on 6s

Hi Guy

I'm sorry, but have need your help

I find in error log on my iPhone 6s 128gb, this error : system memory reset, largest process springboard.

it is the first time that i can see it.

Time of event is 3.30 am, while I was sleep,my phone was in stand by and every app was closed.

Can you say me what is this error?

I have fear that it error is as iOS 8 when springboard restart the device.

thank you

Frank