Hass OS Sleep Mode


in the past I had used Home Assistant on a Synology NAS.
Since the upcoming release did not support USB Devices (zigbett2mqtt Stick) anymore I have Setup a new Installation on an Raspberry Pi 3+.

It works, but I have noticed an anoying bug.
After a while, it looks that the Hass OS in a sleepmode oder standby mode.
When a Sensor (move detection or remote) starts an action there is a delay about 5 to 10 seconds.
when i use the sensor again it starts the action immediatly. After some while it needs seconds again.

it look that there is something going to sleep.

Any Ideas?