Time Machine Backups

Hi -

I haven't been very good about doing backups in the past and I am trying to do full backup using Time Machine before doing a fresh install of High Sierra.

I have an older MacBook pro (late 2011) and don't have the Power Nap mode. I know the process of running the full backup is likely to take 4 or so hours. Do I need to keep watching over it so the machine doesn't go into the sleep mode and thus stopping the Time Machine backup?

Is there a configuration available to prevent the machine from going to sleep so the Time Machine backup continues uninterrupted? (even without the Power Nap option)

Thanks