When dd was 6 we went through an experimental 'choose your own bedtime' phase. I was hoping that dd would listen to her own sleep cues and go to bed when she really needed to vs. us telling her to. Well, it didn't work, and the lack of sleep manifested itself in the form of temper tantrums. We went back to a bedtime, and the temper tantrums stopped.
I always think about diet in these cases too, but in dd's case the culprit was definitely positively sleep-related.
~ Meredith, mom to dd(Jan '02), ds1(May '04) and ds2 (June '07) ~ :