2015 imac not waking up

To put it to sleep I leave it or apple logo > sleep.

If I wake it up within a few minutes, i get prompted with the login screen.

Beyond that, it will not wake up. No power button press or key press will wake it up. Even with patience, I have waited a while and it just does not wake up. I have to hold the power button down until it restarts.

iMac (Retina 5K, 27-inch, Late 2015)

I have a second monitor but it will not wake up even when all peripherals/monitors are unplugged.

I have played around with energy saver settings but with no success. It's the same result every time.