You can use nested IIf expressions to convert the military time settings into the 3 categories:
IIf([MyTimeField]>2259 Or [MyTimeField]<700, "NIGHT", IIf([MyTimeField]>659 And [MyTimeField]<1400, "AM", IIf([MyTimeField]>1399 And [MyTimeField]<2300, "PM", "")))
Or you can create 3 separate...