The number of children age 5 to 6 who go to bed at 10 p.m. or later has quadrupled over the past 20 years, according to a recent survey by an association on child health.

The survey also shows the number of toddlers 18 months to 2 years old staying up late has doubled in the same period.

The survey, conducted in fiscal 2000 by the Japanese Society of Child Health, covered 6,875 children nationwide age 1 to 6 and their mothers. It also found 18 percent of the mothers think they might be abusing their children.

The society has conducted three surveys, at 10-year intervals, since fiscal 1980.

As for sleeping hours, 40 percent of children age 5 to 6 went to bed at 10 p.m. or later in fiscal 2000, compared with 10 percent in fiscal 1980 and 17 percent in fiscal 1990.

Among 18-month-olds, 55 percent fell asleep in that time period in the latest survey, more than double the figure 20 years ago, while 59 percent of 2-year-olds did so in fiscal 2000, a two-fold jump from fiscal 1980.

Meanwhile, 33 percent of the mothers felt they had difficulties in raising children and 8 percent complained of psychological problems in child rearing, up from 5 percent in the fiscal 1990 survey.

Among mothers who think they might be abusing their children, 80 percent said they use strong language, 49 percent said they resort to forms of violence such as spanking, and 17 percent said they think they are too strict about manners.

Officials of the society said that any mother can commit child abuse and that support for such mothers is vital to prevent abuse.