I have 4 DC so am no stranger to lack of sleep and sleep issues. But DD (3) taking the biscuit (a middle one). Ever since she was young she has been a terrible sleeper. She has always had the capacity to lie awake for hours no matter what. As a baby she would lie awake for up to three hours not crying in the middle of the night. We've had various periods have terrible sleep and I've fixed it with being strict (with myself particularly). I've never let her have any sleep crutches. She fed to sleep at night until about eight months but that's it. Sat in her room with her a handful of times to try and help her get to sleep but she has never fallen asleep with me doing this, just lies awake.
At the moment she is waking at some point in the night and staying awake for an hour to two hours. As a result she is exhausted in the day. Last night for example she woke and cried, I went in and took her for a wee and back to bed. Twenty minutes later she cried for me, left her for a bit and went back in. Went out again, forty minutes later crying etc and so it goes on. I know she is just lying awake quietly in these periods and not going back to sleep.
So as not to drip feed:
She watches television rarely, once a week maybe
She gets plenty of fresh air and exercise
She eats well and never has sugar
Her room is dark and quiet and warm.