Increased Calorie Burning
Every time your workout, your metabolism slowly speeds up and then slowly returns to a normal rate. During the slowdown period, you burn calories more quickly than usual. With multiple shorter workouts in a day, you get multiple high calorie burning periods instead of the single period you would get with a longer workout.
After you've been working out for an hour or two, you get really hungry. Putting in that much energy also makes many people adopt an “I deserve this” mentality. They end up eating more than they would normally as a reward for their hard work, which results in a weight loss plateau or even weight gain. Working out in shorter bursts decreases this risk.
Fewer Energy Slumps
Working out for an hour or two at a time often comes with energy slumps. As long as you're drinking lots of water and eating healthy meals or snacks in between the workouts, you'll be able to put in a full effort for each workout session.
If you like the idea of an occasional longer exercise session, but only have time for shorter sessions most days, think about setting up a combination routine. Maybe it makes sense to spend a couple hours at the gym with the weights or in the pool twice a week and then do shorter aerobic routines on your own at home the rest of the week.
The bottom line is that the best workout routine for you is the one that you'll actually do. If it isn't feasible to block out an hour to spend at the gym, look for 20-minute workout routines you can complete from the comfort of your home. If 20 minutes is too much, look for ways to carve out 15 or even 10 minutes from your current schedule, and then build from there.
Ralph Roberts is a nationally certified personal trainer working at the Downtown Athletic Club in Amarillo, Texas. Whether you're looking for an individual or group training program, he has something that fits the bill.