Well when are you trying to do it? It works differently for different people. For some people, it's best to do it after 5-6 hours of sleep, for others, right before bed is best. For me, right when I wake up in the morning after 8 hours of sleep, or just before bed is best. Or even sometimes during a nap. I suspect you fall asleep quickly because you are either not focusing hard enough, or you're just too tired :p