Some people wake at the crack of dawn to exercise, with others waiting until after work in the late afternoon or evening. While exercise is great at any time of the day, it seems our internal body clocks can have a huge effect on the amount of calories that we burn. Finding the perfect time for your fitness regime is part art and part science, with each person needing to match their personal preferences with universal circadian rhythms and innate human physiology.

Circadian rhythms have a profound effect on how your body operates, with our internal clocks affecting things like blood pressure, body temperature, heart rate, and hormones. Your circadian rhythms also affect how many calories you burn, both during exercise and while going about your daily activities. As it turns out, the regular activities that we do on autopilot account for 60 to 70 percent of all the energy we burn, with the daily ebb and flow of our circadian rhythms affecting us all in similar ways.    

According to a new study published in Current Biology, our bodies have an internal clock that sends a message to burn the most calories in the late afternoon and evening. While people often talk about the benefits of early morning workouts, we actually burn about 10 percent more calories from 4PM to 6PM in the afternoon. It's important to note, however, that the study measured resting energy expenditure rather than exercise expenditure, which may or may not be independent of these circadian rhythms. Testosterone levels are also raised later in the day, with PM workouts often recommended for people who are training to increase muscle mass and strength.

While we may burn more calories later in the day, you also burn up to 20 percent more body fat when we exercise on an empty stomach. While this can happen at any time, it's much more likely first thing in the morning before you've had breakfast. Working out in the morning can help to curb your appetite for the rest of the day, with AM routines also beneficial for people who have trouble sleeping. Because exercise increases your heart rate and body temperature, evening routines can disrupt your sleeping patterns. Perhaps the best thing about morning fitness programs, however, is the simple fact that they happen before anything else can get in the way.  

When it comes to setting up specific exercise programs, personal preferences play the most important role of all. While we may burn more calories in the late afternoon, we also burn more calories on an empty stomach first thing in the morning. Whether it's AM or PM, working out when you feel strong and confident is the best way to develop a regular exercise routine regardless of innate circadian rhythms. Consistency is key when it comes to working out, with morning people much more likely to stick to a routine when they're feeling fresh and night owls more likely to stay consistent if it means they can avoid a morning alarm.

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