In order to fall asleep, first one needs to be able to enter into an intermediate state of consciousness between wakefulness and sleeping.

To achieve this, one can resort to passive mediation and self-listening, or to slow motion movement. A specific and very effective exercise is to imagine passing very slowly through a ring, visualizing the parts of the body that go through it one after the other. Start from the toes. After a few seconds, you feel your feet and legs warm up and the sensation of tingling and an increase of weight. Sleep will come in a few minutes, generally before you get your pelvis to pass through the ring.
Doing these types of exercises, it sometimes seems as if you remain suspended for hours between wakefulness and sleeping, while the observer remains awake. You are actually sleeping deeply, and only occasionally do you experience a low level of consciousness. Even so, you feel perfectly rested in the morning.
Gradually, as you acquire experience, you can also learn to relax completely and to fall asleep for about thirty minutes, and thus fully recuperate your energies.