It's no mystery that a diet full of fried foods, giant portions, decadent desserts, alcohol, and sugary soft drinks will lead to weight gain. And there's little question why the pounds pile up when you take in more calories than you burn in physical activity. But how do you explain weight gain when your lifestyle includes regular exercise and a healthy diet that is controlled in calories? Gaining weight is absolutely maddening, especially when you really don't understand why the needle on the scale keeps going up.

"Weight gain is so complicated; there are so many factors that can impact your weight. It is more likely a combination of things more than just one factor," explains Michelle May, MD, author of Am I Hungry? What to Do When Diets Don't Work.

Here are five factors that can cause the scale to creep up when you least expect it.

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1. You Might Be Gaining Weight Because of Lack of Sleep

The body functions best when well rested. "When you don't get enough sleep, your body experiences physiological stress and, biochemically, you store fat more efficiently," says May.

When you're tired, you also don't handle stress as well, so you may reach for food as a coping mechanism. Further, you may be taking in extra calories from late-night snacking. Some people think eating might help them get back to sleep, but all it really does is add more calories to their daily total.

Symptoms that you may not be getting enough rest include fatigue, low energy levels, nodding off easily, and feeling irritable.

Strive to get eight hours of sleep each night.

"Add about 15 minutes to your bedtime and see how you feel," suggests May. "Continue to experiment with additional 15-minute increments until you find the ... amount of sleep that is right for you."

When you develop good sleeping rituals and get regular exercise, you sleep better, she adds.

2. You May Be Gaining Weight Because of Stress

We live in a society that demands we do more, be more, and achieve more. Stress moves us forward and helps cope with life's demands, but it also affects our mood and emotions.

"Stress response, whether it is 'fight-or-flight,' juggling too many responsibilities, or coping with financial pressures, triggers a biochemical process where our bodies go into survival mode," explains May. "Our bodies store fuel, slow down metabolism, and dump out chemicals [cortisol, leptin, and other hormones] which are more likely to cause ... obesity in the abdominal region."