Body composition (the more muscle we have, the higher our metabolisms), protein intake, hormones, stimulant use (like caffeine, which speeds up metabolism temporarily), fasting (which lowers metabolism) and environmental temperatures (i.e. when it’s hot or cold out, our body burns more calories trying to keep itself at a constant temperature) are also considered to have an impact on metabolism, registered dietitian Tristica Curley of Fueling with Food adds.
Skimping on sleep can derail your metabolism. In a study at the University of Chicago, people who got four hours of sleep or less a night had more difficulty processing carbohydrates. "When you're exhausted, your body lacks the energy to do its normal day-to-day functions, which include burning calories, so your metabolism is automatically lowered," explains Peeke.