Specify time of day for backups

It would be nice to be able to specify a general time of day for backups to be made. Something like morning, noon, night. Of course to be able to pick the exact hour would be even better.

Right now backups are being made around 9pm local time on my server and that is making things very slow whenever I work in the admin area of wordpress, which is around 9pm in the evening.

Thanks