Greta Thunberg's father has said he thought it was "a bad idea" for his daughter to take to the "front line" of the battle against climate change.
Millions of people have been inspired to join the 16-year-old in raising awareness of environmental issues.
But Svante Thunberg told the BBC he was "not supportive" of his daughter skipping school for the climate strike.
Mr Thunberg said Greta was much happier since becoming an activist - but that he worries about the "hate" she faces.
As part of the same broadcast, guest-edited by Greta for Radio 4's Today programme, Sir David Attenborough told her she had "woken up the world" to climate change.
She called Sir David on Skype from Stockholm in Sweden, where she lives, and told him how he inspired her activism.
The broadcaster and naturalist told Greta she had "achieved things that many of us who have been working on the issue for 20 years have failed to do".
He added that the 16-year-old was the "only reason" that climate change became a key topic in the recent UK general election.
Greta was nominated for this year's Nobel Peace Prize, after spearheading a global movement demanding world leaders take action over climate change. It led to co-ordinated school strikes across the globe.
She is among five high-profile people taking over the Today programme as guest editors during the festive period.
The BBC flew presenter Mishal Husain to Sweden to interview the teenager and her father.
On the decision to fly, Today editor Sarah Sands said: "We just did not have time for other means of transport. But we met our cameraman there and the interview between Greta and David Attenborough was conducted by Skype, which felt the right way for the two of them to communicate."
Speaking to Husain as part of the show, Mr Thunberg said his daughter had struggled with depression for "three or four years" before she began her school strike.
"She stopped talking... she stopped going to school," he said.
He added that it was the "ultimate nightmare for a parent" when Greta began refusing to eat.
To help her get better, Mr Thunberg spent more time with Greta and her younger sister, Beata, at their home in Sweden. Greta's mother, opera singer and former Eurovision Song Contest participant Malena Ernman, cancelled contracts so the whole family could be together.
The family also sought help from doctors, Mr Thunberg said. Greta was diagnosed with Asperger's - a form of autism - four years ago, something she has said allows her to "see things from outside the box".