Where to find sleep settings on macOS Big Sur
On desktop Macs (iMac, Mac Mini, Mac Studio, Mac Pro), go to System Preferences > Energy Saver. You'll find a slider for "Turn display off after" which controls how long your Mac waits before dimming and sleeping.
On MacBooks, go to System Preferences > Battery > Battery/Power Adapter. Laptops have separate settings for battery and power adapter behavior.
What's different about macOS Big Sur
Big Sur introduced the Battery preference pane for laptops, separating it from Energy Saver. It also introduced the 'Optimize video streaming while on battery' option. Sleep settings are otherwise similar to Catalina.
Why changing settings isn't enough
You can set the display timeout to "Never" on macOS Big Sur, but that's a global change. Your Mac won't sleep on its own until you change it back. On a laptop, that means draining your battery if you forget. And if your IT department manages your Mac, these settings may be locked.
Shake It On on macOS Big Sur
Shake It On keeps your Mac awake on macOS Big Sur by moving the mouse at regular intervals. It works regardless of your Energy Saver settings, including on managed Macs where those settings are locked.
Smart conditions let you control when it runs: only during work hours, only when plugged in, only when a specific app is open. Set it up once and it handles the rest.