Their perfect guide to napping says to aim for 20 minutes and no more. This is really tricky because you can't just set a timer for 30 minutes because the time

Their perfect guide to napping says to aim for 20 minutes and no more. This is really tricky because you can't just set a timer for 30 minutes because the time it takes to fall asleep is variable. Are there any wearables or techniques to solve this problem of starting a timer when you fall asleep?