Hi there Vegemate,
I presume during your day you think about not being able to sleep later that night and it agitates you. When this happens just know it is just a thought at THAT time, be aware of where you are, look at your surroundings, take a deep breath, and go back to whatever it is you're doing in THAT moment. Try to adapt some mindfulness to your day and it will help you come sleep time.
I've had a fear of the night and trouble sleeping for many years, leading to panic attacks and depression. When I went to my course I felt comforted in the surroundings of the retreat center but after night 3 I found myself, like you, and Sid, laying in bed shuffling about and not sleeping until 2 or 3 in the morning. I asked my teacher and he said its normal as its a "deep operation" and for me the first time sharpening the mind and then onto to Vipassana. He told me to focus my attention to the soles of my feet or palms of my hand, and I thought this worked well through the rest of the course.
When you lay down to rest. Maybe lay on your back for 5 minutes or so. Feel your body as a whole energy. I heard this from Eckhart Tolle and I find it to be beneficial. Goenka also talked about meditating from the moment you wake to the moment you sleep, this of course is impossible all of the time, but at times during your day, just be aware of your breath, your body, sensations as you're sitting still or as you move, your surroundings, etc.
Added little story I'd like to share: I wanted to stick to the rules of the Vipassana course rigidly, but my room-mate was using sleep potion to help him sleep. A chemically made liquid he'd dabble on his pillow. I didn't mind him using it, I just felt for me I wanted to let things happen naturally, if I didn't sleep, I wouldn't sleep, if I did, I did. I felt these remedies were but another way to avert reality, and that's really what the course was about for me.
So I had a dilemma: obviously we weren't allowed communicate, but he showed me this little bottle and he made a gesture about putting it on my pillow so that it would help me sleep... because we just HAD to sleep! This was my first encounter with the stuff, so I took it from him, put some on my pillow and gave him two thumbs up. After about 10 seconds of lying down I realized this was but another drug to help me through life, I became a little drowsy with the scent and threw my pillow off the bed. The smell from his pillow was still surrounding the room, I was angry, and stayed up contemplating the issue for hours.
The next day I was just as angry and confused as to what to do. So during lunch I met him in our room and picked up the potion. Got his attention and pointed at the bottle, then made a "meditative stance" through some sort of improvised sign language as to TRY to say "I'm sorry my friend, but I'm here to meditate." He replied "Ahhh, yes" quietly. I'd thought he'd understood where I was coming from, I was delighted with the outcome. That night we finish up our day and I go to my room, the smell is worse than ever. I guess he thought I was trying to say "Thank you my friend, this sleeping medicine has really helped meditation today because I was able to sleep last night." I flip out and leave the room directly upon entering, and decide to go talk to an assistant teacher, I explained the situation to him and he explained it to my room-mate, he was understanding and I was thankful for this. I didn't want any quarrels.
On the 10th day when we're allowed speak. I brought up the topic lightly and he seemed understanding but he didn't see the funny side of the situation as I did. Then he left without saying goodbye. I thought there was a moral from this added experience on the course. I encourage you to continue your practice and not use anything but nature to help you sleep. I think Sid's post above can really help you. Try not to fall asleep, and you will be successful. Sorry this is very long winded. I'm fascinated by the effects meditation has on each individual, and I know how annoying sleep deprivation can be. Very best of luck with everything.
Gary