dumb down

やはり睡眠不足はダメですね。

Lack of sleep blights pupils' education
Sleep deprivation is a significant hidden factor in lowering the achievement of school pupils, according to researchers carrying out international education tests.

It is a particular problem in more affluent countries, with sleep experts linking it to the use of mobile phones and computers in bedrooms late at night.

Sleep deprivation is such a serious disruption that lessons have to be pitched at a lower level to accommodate sleep-starved learners, the study found.

The international comparison, carried out by Boston College, found the United States to have the highest number of sleep-deprived students, with 73% of 9 and 10-year-olds and 80% of 13 and 14-year-olds identified by their teachers as being adversely affected.

In literacy tests there were 76% of 9 and 10-year-olds lacking sleep.

This was much higher than the international average of 47% of primary pupils needing more sleep and 57% among the secondary age group.
＜中略＞
"But worse than that, teachers are having to modify their instruction based on those children who are suffering from a lack of sleep.

"The children who are suffering from a lack of sleep are driving down instruction."

That means that even the children who are getting enough sleep are still suffering from this sleep-related dumbing-down.
http://www.bbc.co.uk/news/business-22209818

dumbは「ばかな；頭の悪い；（物が）つまらない」（『ジーニアス英和辞典　第４版』大修館）という意味を持ちます。そしてdumb downで「…を大衆化する；〈 教育など〉の程度を下げる」(ibid)といった意味になります。「学習のレベルを下げる」といったところでしょうか。

睡眠不足は学習の敵ですね。私も日々の学習のために早寝早起きを心がけます。(Hatahata)