Question: Does this sound right? Is the grammer correct? I used to stare at the pictures, until eventually I became sleepy; a trick that was very useful when I used to be too excited to sleep if Christmas was coming, or my birthday!

Answer: I don't think it is exactly incorrect, but it doesn't read very well. I think you have the sentence the wrong way around. I would suggest re-writing it as follows:\n\n"If I was too excited to sleep because Christmas or my birthday was coming, then I would stare at the pictures until I eventually became sleepy. It was a very useful trick!"
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