"I'll have to admit I slept well for the first time in a long time last night," she said at Hawke's Bay airport, smiling broadly.
Ms Ardern said she had no regrets about her decision, as tough as it was to reach.
"There's still a range of emotions. I of course feel sad but also I do have a sense of relief," she said.
"I don't doubt the decision."
World leaders have paid tribute to Ms Ardern, who will leave the job as soon as Labour selects a successor.
Australian Prime Minister Anthony Albanese penned an opinion piece in Nine newspapers, crediting her with writing a new rule book for leadership.
"Through the sheer power of her example, Jacinda Ardern has reminded us all that kindness and strength are not mutually exclusive. Even more importantly, she has shown that a true leader possesses both," he wrote.
Ms Ardern kept her decision secret until lunchtime on Thursday but offered Mr Albanese a heads-up prior to her announcement.
"I sent him a message before making the announcement and that's because he is one of the leaders that I do have a close relationship with," she said.
"I deeply valued the work that we've done together and the time we've had.
"He's become a friend and I respect him greatly. So I sent him a message. We had a quick conversation afterward yesterday and he only had roots of kindness and support."
United States President Joe Biden credited Ms Ardern with growing ties between the two countries.
"The US-New Zealand partnership is stronger than ever thanks in large part to your leadership," he said.
"Your stewardship in advancing a free and open Indo-Pacific was crucial."
Ms Ardern said she was taken aback by the volume and depth of support she had received.
"I actually feel deeply humbled," she said.
"To have those messages of gratitude has been really moving for me and for my family."
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