If you're looking to lose weight, morning may be best. For one thing, some people swear by "fasted training," where you exercise right after waking up and before eating breakfast, either because cardio on a full stomach causes them discomfort or because they've heard that it helps burn more fat. The science on that second part is mixed: Some studies show that fasted training is better than "fed training" for controlling appetite and improving glucose tolerance and insulin sensitivity. Other studies have shown that whether people exercise fed or fasted has no impact on their weight loss when everything else is equal. It's up to you to decide whether an empty stomach or a bit of fuel in the tank helps you get the best workout in.
For most of us, morning may also be the best time for maintaining a consistent workout schedule, since most last-minute obligations pop up in the evening and a long day at work can sap your willpower to work out. And anyone who's gone to the gym at 5:30 p.m. knows that there's a definite rush period after work. Going in the morning could mean the difference between a quick and satisfying session on the treadmill and an aggravating wait in line.
Many people believe that working out at night makes it harder to get to sleep. Your mileage may vary, but according to scientific research, there isn't much truth to that. Studies have shown that for both cardio and weightlifting, the time of day you work out doesn't make much of a difference in your sleep quality — just the fact that you exercise at all helps you get that precious shut-eye. Of course, sometimes you plan a morning workout only to bargain with yourself over whether to sleep in or get up. We've got an article for that.