Here's another method.

DECLARE @StartVarchar(5) = '07:00',

@EndVarchar(5) = '05:59'

SELECTCASE

WHEN REPLACE(@Start, ':', '') < REPLACE(@End, ':','')

THEN DATEDIFF(HH, CAST('1900-01-01 '+ @Start AS SmallDateTime), CAST('1900-01-01 ' + @End AS SmallDateTime)) + 1

WHEN REPLACE(@Start, ':', '') > REPLACE(@End, ':','')

THEN DATEDIFF(HH, CAST('1900-01-01 '+ @Start AS SmallDateTime), CAST('1900-01-02 ' + @End AS SmallDateTime)) + 1

END

I don't know if you want the difference in minutes or hours, but I assumed hours. Other wise change HH to MI in the DATEDIFFs and remove the +1's.