Esteban Ocon has revealed that his schedule in 2019 left him physically "destroyed".
The Frenchman, who returns to the grid with Renault this year, was Mercedes' reserve driver in 2019.
He says he was worked particularly hard.
Crazy travel plan
The 23-year-old said: "It was very, very demanding. I sometimes did not sleep for two days and the travel I was doing was quite crazy."
Ocon said he was not merely on the bench in 2019, but constantly rushing between the simulator at Brackley to PR events and "wherever the car was driving".
He smiled, saying: "They used me well, but a lot. So after the testing in Abu Dhabi, I arrived at my training camp the lightest I've ever been. I was destroyed, basically, from the year."
However, he says he has recovered now.
"He concluded: I've put on four and a half kilos of muscle, which is good in two months."