Insomnia again. Mindful meditation, a relaxation protocol, and soothing white noise can work. Making sleep happen is tough. It is about letting go. Refrain from evening exercise, caffeine, alcohol, video games, or heated discussions. Keep pesky pets in another room and sleep in a separate room if bedmate snores or is very restless. Short of a thorough history by your md, melatonin, valerian root, or 5 htp "so so".
Ultradian cycle. You mind has a 90 minute cycle that is running all the time. When it is in the rising phase you will not fall asleep, no matter what (unless you use powerful sedatives). In the descending phase you can fall asleep if you are not too stimulated. The best bet is to have a stable sleep/wake cycle and then put yourself in a low stimulation environment (like reading in bed) and wait until sleepy.