If iCloud Drive fills up overnight, the cause is usually the Desktop & Documents sync feature, which uploads everything you drop on your Desktop.
Short answer: disable Desktop & Documents Folders in the iCloud settings, then move the files that used to live in iCloud back to your local Desktop manually.
What the feature actually does
macOS offers to sync your Desktop and Documents folders to iCloud Drive. Enabled once, it quietly ships every file you drop on the Desktop — including 10 GB project files — to Apple's servers.
Convenient across devices. Catastrophic for a 50 or 200 GB iCloud plan if you work with big files.
Turning it off safely
- open System Settings
- click your Apple ID at the top
- open iCloud > iCloud Drive
- turn off Desktop & Documents Folders
- confirm the warning — the Desktop will look empty for a moment
The files are not lost. They now live only inside iCloud Drive.
Getting the files back locally
- open Finder
- in the sidebar, under iCloud, click iCloud Drive
- open the Desktop folder
- select everything and drag the files out of iCloud Drive and back onto your real Desktop
- repeat for Documents
Once the files are on the local drive, iCloud space frees up.
Better next routes
If iCloud is still full, the next likely culprit is Photos. Continue with Best App to Find Duplicate Photos on Mac (Photos Library).
For the broader Mac framing, use What Is Purgeable Storage on Mac?.