Using such long intervals defeats the purpose of the salt cube timer. The idea is to do short intervals that are random. This prevents you from falling too deeply asleep and by them being random tricks your mind into becoming conscious during one of the longer intervals. Also waking up during non-rem periods won't do you much good, which is what will happen if you do a bunch of 40 minute intervals. The idea is to start the timer ramp on the longest rem cycle.