I'm not very experienced in AP, I apparently used to be able to naturally when I was younger but I don't know if what I did was AP.
Anyways, when I am tired and am laying in bed, sometimes I get this sensation that I am falling. I immediately get scared and try and open my eyes and move around. It's always tough to force movement when this sensation happens.
I am wondering if this is somehow connected to astral projecting because it always happens when my mind is awake and my body is tired, which I read is a good condition to be in when attempting to project.
Actually that feeling is common to most people. Me and my friends feel that same sensation usually when we are thinking before going to bed ,then suddenly it feels like you've been shaken or your falling. I use to AP as a child to so it might be a weird coincidence. i'm not really an expert but i'm just tell you my own past experience.
Like you said, it can happen when your mind is awake but when your body is trying to sleep. I personally tend to have a spinning sensation more than a falling one. It gets stronger the longer you focus on it. As for what it really is, that is up to you to decide. I personally believe it's a part of our mind that has been ignored the last 2000 years or so. Religions used to be based off that stuff, man. Crazy.
I experienced this as a child, and it has moved more to a quaking sensation, as in my mind shifts violently, once side rising high as the other lowers, then vice versa. It corresponds with the tiredness though.
AP does sometimes feel a little like "falling" because the "astral" body is "lifting" from the physical. However,the Astral body is always "connected" to the physical. Often described as a "silver or golden cord".
Its best to meditate, or perform something like yoga or tai chi before sleeping. The falling or spinning (I get spinning). Is due to being very active right before sleeping, drinking caffeine, smoking, exercising, or doing anything that gets your blood pumping. When you lay down, the blood that was chilling in your legs has an easy ride up to your brain. And it tends to make you woozy. If you do exercise before bed, do some relaxing stretches and breathing to calm it all down before turning in for the night.
Funny enough it mimics the same feeling of leaving the body during AP.
So it could be OBE (out of body experience). AP is generally described as purposefully traveling. OBEs are involuntary/unconscious. You can even OBE slightly while awake, which causes you're mind to keep drifting off, making you act ditzy.
Ground and you'll go back to normal.
If it happens a lot while you are awake you may need to contemplate why you're attempting to escape your life, and correct that. Then ground again =)
OBE while sleeping is perfectly normal. I do it most nights.