digital input are boolean (ON/OFF) type only. DM is a 16-bit integer. You can't set a boolean type to an integer type.
However, if you use HMI to set a DM value and you want to decide whether to turn ON/OFF an internal relay named "Night", then:
IF DM[1] = xxx
SETIO Night
ELSE
CLRIO Night
ENDIF
This would work. You can't really change an input bit for more than a single scan cycle. So it makes sense to SET/CLEAR an internal relay and use the internal relay in the ladder logic to trigger other part of the program.