Sleep seems hugely safe. It might error a transfer in progress, but it retries number-of-retries times.
There was a time when I deliberately slept my computer in the middle of backups to test things out.
How does Duplicati Resume After Interruption? is what should happen on more severe interruption.
Not only should it give you a synthetic backup “version” of what is done, but it will (AFAIK) continue interrupted backup from the point of interruption (not from start). Have you seen an actual problem?
There are a few open issues where things get confused, Duplicati says to Repair, and Repair works.