Young children who don\'t follow a regular bedtime may have their brain development stemmed, researchers in London suggest. Senior author Amanda Sacker, director of the ESRC International Centre for Lifecourse Studies in Society and Health at the University College London, said sleep is the price to pay for the investment needed to allow learning the next day. \"Early child development has profound influences on health and well being across the life course,\" Sacker said in a statement. \"Therefore, reduced or disrupted sleep, especially if it occurs at key times in development, could have important impacts on health throughout life.\" The study involved more than the 11,000 7-year-olds, all of whom were part of the UK Millennium Cohort Study -- a nationally representative long term study of British children born from Sept. 2000 to January 2002. The research was based on regular surveys and home visits made when the children were ages 3, 5 and 7. Irregular bedtimes were most common at the age of 3, with 1-in-5 children going to bed at varying times. However, by age 7, more than half went to bed regularly between 7:30 and 8:30 pm. At age 7, girls who had irregular bedtimes had lower scores on all three aspects of intellect assessed, but not in 7-year-old boys. Irregular bedtimes by the age of 5 were not associated with poorer brain power in girls or boys at the age of 7. But irregular bedtimes at age 3 were associated with lower scores in reading, math and spatial awareness in both boys and girls, suggesting age of 3 could be a sensitive period for cognitive development, the study said. The findings were published online in the Journal of Epidemiology and Community Health.