This might sound very esoteric and hard to grasp, but this is something that I have noticed in myself that tells me I am falling alseep, and it allows me to in a way make me fall alseep.
When I am about to conk out, my vision gets very bright. It is similar to what you see if you close your eyes, and press on them hard with the side of your fist, after 5 seconds or so, you see flowing undulating shapes. Only for when I am falling alseep, it is very bright light. So bright that I could mistake it for a light in the room. It moves and changes, over time. Interestingly, if I try too hard to focus on it, it "flashes" and goes away until I stop trying to think about it. Eventually it comes back. I have found that around 5 minutes after noticing this, I will have fallen alseep. I truly have no idea what this is (and it is completely unrelated to the lights I see durring the day. For those of you who don't already know, I see lights), but it helps me sleep in a way. Because it tells me I am close to it, and I can also "remind" myself that I will see this light before I go to sleep, and it will come later on.