I just posted this on another thread and when I ran into this thread I realized I posted this info in the wrong place!
I would like to share my astral experience with you, I hope you don't mind. Some years back I was going through depression which made me want to avoid the world around me and just sleep all the time. After large quantities of sleep very strange things would happen visually and physically. I wouldn't be able to move my body and would start to panic, then I would float up towards the ceiling slowly, all the while crying, freaking out badly and fighting this invisible thing with all my might. The more I faught, the faster I would float upwards closer to the ceiling, not being able to control my limbs at all. I felt like I was going somewhere never to return and it was a horrifying feeling. I had these all the time, most of the time accompanied by frighting noises and faces and very tingly sensations. I would try so hard to come back into my body and force myself awake, but a "force" seemed to be pulling me back into that state and it was very strong. Finally I would suck myself back in, and that's exactly what it felt like, getting pulled or sucked back in to the point where my body would jerk like it just got dropped onto the bed.
This would happen regularly to the point where I was afraid to go to sleep. Then one day I came across a book about astral projection and it had all kinds of stories about other people's experiences with it. I couldn't believe it! Same exact types of experiences. After that, I saw these experiences completely differently. The scary parts didn't exist any more. I learned that during these experiences your mind may create dreamscape and in my panic, I created a very negative one, hence the scary faces and noises. I grew confident that when my "astral body" left my real body, it would actually be able to come back and not just go away never to return.
First thing I did when I had another one was look down at my limbs. It was so wierd, I could see my astral limbs. They didn't look like my own arms and legs, more like an illuminated energy shaped vaguely like limbs - they weren't solid.
I think I'm going to stop here because I don't want to bore everyone with a long story (too late).
The gist of it is, I believe everyone separates at least slightly from their real body during a sleeplike state. I think you need to learn how to use your astral body which is quite different then controlling your earthly limbs when you are able to achieve awareness in this state. You have to separate your mind from earthly concepts like solid matter because in this state you are an energy that should probably be able to pass through matter easily, since everything is just engery (does that make sense?). I am by no means an expert on this subject, I just have my experiences to go by. But I'll tell you, it was a beautiful time for me that gave me answers to some of my life questions.
And in answer to your question, when I had many of the early astral projections, it was daytime. I think when you force your body to sleep and your not really tired you may have a chance to enter this state of conciousness, not in the regular sleep state where your body and mind just want to rest and recouperate. I hope I was able to offer something of help to you.