Yes you can leave your laptop on overnight to download or upload files.
Just make sure to keep the laptop from going into sleep mode overnight as when the laptop goes into sleep mode it will stop uploads or downloads.
You can set the screen to go off or hook a separate monitor to the laptop and then switch the laptop screen off and then turn the other screen off as well.
Windows can sleep while copying files but the files can stop transferring when windows sleeps.
Sleep mode means pausing all applications and processes and putting computer on a low power consuming state by turning off processor.
Running an application needs the processor to be active. If you want to save power while still downloading something, just turn off your monitor.
Files will not still download in sleep mode.
All downloads will stop when your laptop or other computer enters sleep mode.
You will need to set up your laptop to keep it running even when your lid is closed for the downloads to continue.
Your computer does not still upload when in sleep mode.
Uploads will be interrupted and fail if your PC goes into sleep mode during upload.
When your computer goes into sleep mode it stops activity such as uploads, downloads and internet connections.
So if you need to keep your computer on to upload files or download files then you can set the laptops screen to go off but still keep the computer on and out of sleep mode.
With a desktop you could just leave the computer on and shut down your computer monitor.
The application will resume uploads even if you shut down during a sync operation.
Files sent there will upload as Internet is connected.
If however you do so from a browser this may not be the case.
Some systems may view this the upload as activity and thus not sleep until the upload is complete.