If you have trouble getting to sleep quickly, whether at night or at nap time, try this little mind hack: Imagine your body feeling like it does when you're completely, going-to-pass-out tired.
Zane Claes writes on his Life by Experimentation blog that he's trained himself to fall asleep in two minutes or less -- down from the typical 15-30 minute range -- by using visualisation:
What I mean is to engage your mind in thinking about the physical sensation of tiredness. Not falling asleep, not how nice it would be to sleep, not the next day. I mean visualising the physical sensation of tiredness. Do your eyes droop and your mouth hang open? Do your arms feel heavy and useless? Do you feel like you're sinking into your bed and falling into darkness? Research has shown that the simple act of smiling can make us feel more happy, visualising sports events can aid athletes' performance, and so on. Why not put this power to use?
I like to think of a weight pressing evenly across my entire body, as if some unseen hand was "pushing" my consciousness out of me and into the depths of the bed. Think of the phrase "falling asleep:" there's a reason the verb "falling" is used. We've all felt the sensation, and the power of the mind-body connection is such that simply imagining it is enough to help induce it.
He notes that this technique would likely be easier for those who meditate or use visualisation. It's definitely worth a try to avoid the spiral of not being able to sleep then stressing about when you have to wake up, which keeps you up even longer. Alternatively, you could do some maths.
Sleep Hack: Fall Asleep in 2 Minutes or Less [Life by Experimentation]