screen saver, display sleep, and machine sleep, and deep sleep, are all different things.
Screen Savers are usually simply an animation, originally designed to prevent burn-in on CRT's (though occasionally they caused it, thanks SETI@home), though probably the most famous screen saver software, After Dark, included games in some of their later releases. They range from simple to complex, and use CPU or GPU cycles accordingly. They do not interrupt anything, except for poorly coded video applications. There is no power savings.
Display sleep turns off the display. That's it. It will not interrupt anything, including all video applications. There is commensurate power savings.
Machine sleep or simply "sleep" (equivalent to Standby or Suspend on Windows) puts the machine in a low-power mode and pauses the CPU. The only activities that may take place during sleep are RAM refresh, and the blinking light. There is significant power savings, and sleeping laptops may remain asleep for days before running out of battery power.
Deep Sleep, or "safe sleep" (Hibernate on Windows) saves the contents of RAM to disk, and turns off the machine. There is no activity at all, and no power goes to the machine.
Set up a screen saver or even set display sleep if you like. It won't interrupt your upload.
