Backup my system when it started sleeping?

[surfbug]

I tried to backup the system in the external hd but every time my computer would be sleeping :-}. How do I keep my imac computer on as I back up my system?

 

[Brian33]

Are you using Apple's built-in backup solution, Time Machine, to do the backups, or some other program? I believe Time Machine will delay system sleep until the backup is done.