Hello!
My name is David N. I am a Windows fan and advise people here with Windows questions. I will be happy to assist you with your issue.
When a computer goes into Sleep mode, it turns off output to any monitors. The monitor's behavior after that point is dependent upon the monitor. Sometimes they will turn off entirely, but many will go into a standby mode. In that mode they may or may not turn off their backlight.
It is possible that your monitor has settings for what to do in these situations. You may be able to change its behavior so that this doesn't occur anymore. Unfortunately, this could also result in the monitor shutting down entirely, requiring that it be turned back on manually when you leave sleep mode.
I hope this helps! Please let me know if this resolves your issue or if you require additional assistance.
Regards,
David N.