Sleep modes generally are not supported by pro audio devices and environments and ATOM SQ is not different. While ATOM SQ will enter sleep mode if it no longer detects a host, its operation post-wakeup depends on which sleep mode your computer supports and uses.
Generally if your PC enters the Hibernate sleep mode, also known as S4 sleep mode, you will need to disconnect and reconnect the ATOM SQ in order to resume sending MIDI messages. Other sleep modes, such as Standby (S3), do not require a reconnect. To learn more about sleep modes in Windows, refer to the following article: