There is a blog post here about How to Fall Asleep Fast. The technique comes from the Second World War and making WW2 pilots get enough sleep. A person called Bud Winter was tasked with training pilots to fall asleep quickly. The trick is to physically relax and then mentally relax. If you can keep your mind clear of thoughts for 10 seconds, apparently you will be asleep. 96% of pilots who had been trained on this technique by Winter were able to fall asleep within two minutes or less.