Parents should ban children from using tablets and smartphones before they set off for school in the morning, an expert has said.
Clinical psychologist Linda Blair said use of such devices early in the day could harm concentration in class.
Speaking at the Cheltenham Science Festival, she also suggested teachers minimise the amount of homework that needed to be carried out on computers.
She said parents could set an example by limiting their own screen use.
Mrs Blair, author of The Key to Calm, said requiring children to do homework on computers itself contributed to children becoming tired and unfocused.
Computers and other screens emit a blue light that has been shown to reduce levels of melatonin - a chemical produced by the body that aids restful sleep.
They are also said to raise the levels of the stress hormone cortisol, which makes it harder to concentrate.
She said: "It makes me mad because a lot of schools are doing screen homework. It's so stupid - I wish they wouldn't do that."
She acknowledged that limiting screen use "was not a popular move", but added that it could transform things for young people.
"I encourage people to try it for a couple of days."
Mrs Blair also encouraged families to "establish one point in the day where the family focuses on each other".
"That used to be called dinner. All screens are off and you actually talk to each other.
"They will moan but 10 years from now they will remember it and thank you.
"You have to set limits. Kids have got to know how to manage their screens and be in control."