iMac reboots when in sleep mode

My late 2015 iMac keeps restarting (mostly at night) when it is in sleep mode.

It reboots with a C-chord, followed by another c-chord approximately 5 seconds after the first one.

I have done a complete clean reinstall of High Sierra including a manual file transfer (not time machine restore).

There is no sleep schedule set in system preferences.

There are no messages like" Your Mac shut down because of a problem".

I went to my local mac dealer and he said he could not help me or accept it for repair (apple care still valid) because according to him this is a known software issue.

Is anyone else experiencing this since updating to High Sierra?

Has anyone found a solution?

Is anyone aware of Apple working on a fix?

Michael

p.s. my macbook pro does not have this problem.