timemachine stops backing up during sleep

Computer Sleep is not necessary with the new high efficiency silicone Mac's.

As a test:

Turn on "Prevent automatic sleeping when display is off" and only allow the display(s) to turn off.

Leave the Mac ON during the day and set the display(s) to turn OFF after 10 or 15 minutes.

Do not manually Sleep Computer from the dropdown menu during the day.

Shut down for longer periods of non-use like when your away for more than a day.

You're welcome.

What external drive are you using for Time Machine..?

Depending on the external drive and how it is connected the the Mac, manual computer sleep can cause the USB port to power down without properly ejecting some external drives.

Desktop or Portable and how old is it..?

Budget portables drives tend to have a much shorter life span than Desktop drives.

I would test the WD for awhile without putting the computer into full sleep mode.

Then if the WD acts up, I would set it aside and setup a different external drive for Time Machine.

How full is the Time Machine backup..?

Control (right) click on the Time Machine icon and select Get Info.

Does it backup when you select Back Up Now from the Time Machine dropdown..?

It if has plenty of space and backs up, then perhaps it was just a glitch or a loose the connection.