For me, the following works: day by day I was adjusting the nap length until I found a sweet spot. If the time is too short (30m), then I get woken up exactly when I'm falling asleep and that feels horrible. If the time is too long (60m), then I can't really wake up as I'm already in deep sleep. But, somewhere between those two there is a number which works really well. You just have to find yours.