Songfacts®:

• Hilary Duff's fifth album is named after this song, which she recorded with New York-bred singer-songwriter Matthew Koma. Duff told Entertainment Weekly that the title holds some personal meaning to her. "It stuck with me," she said. "Over the past few years of my life it's just been something that really helped me - just taking a deep breath in and letting a deep breath out. It can be a good thing, it can be a struggle, it can be a power thing—it's just a good reminder for everyone to remember to do that. It's a relief."
• Taking a deep breath in and letting a deep breath out is a form of relaxation, which when practiced regularly, may lead to the relief or prevention of symptoms commonly associated with stress. Here are some fun facts about breathing from The Encyclopedia of Trivia:
  
  In one breath, an adult inhales one pint of air, although the actual capacity of a lung is ten pints.
  
  The average person takes between 17,280 and 23,040 breaths a day.
  
  We lose half a litre of water a day through breathing. This is the water vapour we see when we breathe onto glass.
  
  About 80% of people breathe through only one nostril. Which nostril, left or right, can alternate on average every two-and-a-half hours.
  
  You can not kill yourself by holding your breath.
  
  When Thomas Edison died in 1941, Henry Ford captured his last dying breath in a bottle.
• Hilary Duff performed "Breathe In. Breathe Out" live for the first time on May 14, 2026, during her set at the El Rey Theatre in Los Angeles for SiriusXM's Small Stage Series.