The data also showed that as the children grew from age three to age seven, the behaviour problems — including hyperactivity, conduct problems, problems interacting with peers and emotional difficulties — worsened among those whose bedtime varied.
"What we've shown is that these effects build up incrementally over childhood,” said Yvonne Kelly, and the lead author of the study, in a statement Monday.
“But our findings suggest the effects are reversible … children who change from not having to having regular bedtimes show improvements in their behaviour.”
The results of the study were published Monday in the journal Pediatrics.
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Kelly, an epidemiologist at University College London, and two of her colleagues analyzed data from more than 10,000 children in the UK Millennium Cohort Study, a longitudinal study that is following the lives of 19,000 children born in the United Kingdom in 2000-01.
When the children were three, five and seven years old, their mothers were asked whether the children always, usually, sometimes or never went to bed at a regular time on weekdays during the school year. The mothers were also asked to complete a questionnaire designed to identify behavioural difficulties in children.
Overall, about one in five children didn’t have a regular bedtime at age three, but that had dropped to less than one in 10 by ages five and seven.
The researchers found that seven-year-olds who didn’t have a regular bedtime had more behavioural difficulties than those who had a regular bedtime. The longer they had not had a regular bedtime, the worse their behavioural scores were.
Irregular bedtimes linked to poverty
The relationship held even when the researchers took into account the fact that children from poorer households, those whose parents were less educated and those whose parents had poorer mental health were the least likely to have regular bedtimes.
The researchers suggested that not having a regular bedtime could affect children’s behaviour by:
- Disrupting the children’s natural biological clocks or circadian rhythms.
- Resulting in sleep deprivation. They noted that sleep is thought to be important for helping the parts of the brain that regulate behaviour to mature.
The researchers noted that gaining the ability to control behaviour is a “central developmental process” of early childhood, and they expressed concerns that the effects of irregular bedtimes during childhood could affect the children their entire lives.
However, the reversibility of the effects means it may be possible for health-care providers to intervene.
For example, the paper suggested, screening for disruptions to children’s bedtime schedules could be a routine part of medical checkups.
However, it noted that when parents work long hours or irregular shifts, it may be difficult for them to maintain a healthy family routine so new government policies may be needed to help them.