I am guessing it must have something to do with how fast you can fall asleep and enter deep sleep.
The brain cycles through the various stages of sleep, on average 90 minutes long (it varies from person to person, could be closer to 75 or 105 minutes).
If you wake up in the middle of the cycle, when you're in DEEP sleep, you'll feel very tired and be tired for a long time after.
I find with a long nap past 30 minutes, it's just more common for people to end up waking up in the middle of their sleep cycle. NASA did a sleep study that found out that 24-25 minutes of actual sleep was the sweet spot for taking a nap.
So basically you could set an alarm for your estimated time to fall asleep + 25 minutes.
Or you could do a "coffee nap", you drink a cup of coffee and then immediately go to sleep. It takes about 30 minutes for the caffeine to hit your bloodstream. That way you'll wake up naturally.
Im asking because I cant fall asleep in 30minutes
If it takes you longer than 15 minutes to fall asleep, you might need to look at cutting caffeine or other stimulants out of your diet completely. Exercise will also help.
Usually helps if you're feeling a strong sleep inertia signal (i.e. desire to sleep) compared to just feeling fatigued.
That wont work unfortunately, I am prescribed some. However I had issues falling asleep before the prescription as well