Really though, if you nap for an REM cycle you'll be fine or two or three, it's waking up in different parts of the sleep cycle that's truly problematic for sleep. That's why strict alarm clocks aren't the greatest, but there are now apps that you can use where an alarm will go off during a preselected range of time say I need to be up at 7:30 I can set it for 7:15-7:45, or if I absolutely can't be late, 7:00-7:30 and it senses when you're in your lightest part of sleep (after your deepest REM sleep) and will go off then.