I've used my block heater overnight several times and I think it's ok to do it once in a while. Unfortunately, block heaters can get so hot that they'll burn the additives in the coolant. Eventually enough crap will crust and harden on the block heater that it will seriously diminish the heater transfer ability of the heater. Therefore it's probably not a good idea to make a habit of keeping it plugged overnight everynight, especially when 1.5 hours is usually enough.