Hard Disk Monitoring

I was just wondering if it's possible for an external hard drive to be harmed in any way if my computer goes into sleep mode at any time during an extended self-test.

These things usually take a little over 8 hours to complete for me, and I'm obviously not always going to be rooted to the computer during the entire scan, so I just wanted to be sure that no complications can arise from letting my PC enter sleep mode while the extended self-test finishes running on my external drives.

Thanks for your time and help.

No, generally there should be no damage of the drive when the computer goes to sleep mode during the self test.
The self test may abort in this case (so may need to be restarted) but generally there should be no problems with the drive and the stored data, the extended self test is completely safe.