What does Windows 10 write to my drive before going to sleep?

I imagine that Windows is writing something on the SSD before going to sleep.

That is correct. If there is any data waiting in the disk's write cache, it must be written to the disk before the disk is powered off.

Note that disk caching can be disabled so that everything is written to the disk immediately. This ensures that data is not lost as a result of a power failure, but it does reduce performance.

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Turn Disk Write Caching On or Off:

1. Right-click My Computer, and then click Properties.
2. Click the Hardware tab, and then click Device Manager.
3. Expand Disk Drives.
4. Right-click the drive on which you want to turn disk write caching on or off, and then click Properties.
5. Click the Policies tab.
6. Click to select or clear the Enable write caching on the disk check box as appropriate.
7. Click OK.

Source HOW TO: Manually Turn Disk Write Caching On or Off