13 surprising reasons you’re always dog tired – and what to do about it

Guardian journalism is independent. We earn a commission if you buy something through an affiliate link. Learn more. Feel lethargic even after a full night’s sleep? From not enough fibre to too much sitting, experts explain why your get up and go has got up and gone The very air you breathe could be an issue. “Close the door, shut the window, add a partner and a heater, and by 3am a typical bedroom sits at 1,500 to 2,500 parts per million of carbon dioxide,” says Dr Jemma King, author of Sleep First: Sleep Smarter, Think Sharper, Feel Better. “Research shows that above 1,000 ppm sleep efficiency drops, deep sleep reduces and salivary cortisol rises on waking. You’re sleeping, but your nervous system is mildly activated all night.” Luckily, it’s an easy fix: open a window – even 2cm will do the job, according to King. This supplement isn’t just for bodybuilders. What we think of as “energy” is a molecule called adenosine triphosphate (ATP), produced by mitochondria. It stores and then delivers energy to cells. As you age, you have fewer mitochondria, so your body struggles to produce enough ATP. Creatine can help. “When creatine is combined with a phosphate