New research has shown what many parents already know: regular bed times improve children’s behaviour. Kids thrive on routine, & it has both physical and psychological benefits. Knowing what to expect and when gives children comfort, security and a sense of control over their world.
10,000 children aged 3, 5, and 7 were studied. The data collected included the time that the children went to bed as well as reports from parents and teachers regarding behaviour.
The results showed a clear and significant statistical link between regularity of bedtime and behaviour. During progression through early childhood, those kids that did not have a regular bedtime had worse scores for behaviour. If the children then switched to regular bedtime, their scores improved.
The effects of irregular bedtimes built up incrementally over the course of childhood. Children who always had irregular bedtimes scored worse for behaviour than those who had regular bedtimes at one or two of the ages when they were measured.
The study found that irregular bedtimes were most common at age 3 but by age 7 more than half of the children went to bed regularly between 7.30pm and 8.30pm. 
Irregular bedtimes disrupt the body clock causing sleep deprivation and adverse effects on the developing brain. Given the impact of early childhood on later development, implementing some regularity in bedtime routine should be done as soon as possible.
 Kelly. Y et al. Changes in bedtime schedules and behavioural difficulties in 7 year old children. Pediatrics. 2013 Nov;132(5):e1184-93