What worked for me was to build up gradually over time. I started only napping when I was very tired, in my bed, with the lights off. Even then it took practice before my body learned to fall asleep in a reasonable amount of time.

Then I started napping with the lights on, and then when I wasn't super tired.

Today, I can fall asleep quickly in a chair in a noisy office.

I've really struggled with sleep schedules recently (varying from getting up at 6am to 3pm within a week, due to various late-night engagements whilst trying to get up earlier).

I have found that setting an alarm and counting down from 10 to force myself out of bed unnaturally early has left me feeling really awake once showered and very dozy around mid-day. Time will tell if this is sustainable but it feels pretty amazing right now, far more productive hours (in amount and intensity) than when I was sleeping 8-9hours a night.

Please notice that it also depends on the length of your nap. I do 20 minutes naps and you don't fall asleep in those. Still it's enough to change your brain wave and to get the benefits of the nap.

Also, as you do it every day at the same time, your body gets used to it and you nap faster.

Perhaps you could just try meditation instead.

The 'sleep' part is not actually what's important. Just lie down in a quiet, dark place and close your eyes for 20 minutes, and you will gain a large portion of the same benefits that you'd have gained if you actually fell asleep.

I can fall asleep at pretty much any given hour of the day... unless it is the actual time in the evening at which I should go to bed. I had a terrible case of getting a "second wind" rush at night around midnight and even worse after 3 or 4 am.