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Her new book, "The Scientific American Day in the Life of Your Brain," tracks what's happening in your brain during the average day -- the best food to fuel you through your waking hours (she recommends complex carbohydrates and lean protein; "be wary of fad diets that focus on one food type or group. These can wreak havoc by upsetting your body and brain balance"), when the peak time for exercise is ("Some research shows body temperature may peak around late afternoon or early evening, so that might be the best time to work out" but we are individuals so we'd "do best to keep a kind of daily energy diary" to see when we're at our personal best) and more in the interview below.

That's Fit: How does self-control suck your energy?

Judith Horstman: Like most activities, self-control takes energy, and that requires fuel. And yes, not doing something is an activity. A part of your brain -- usually the reasoning thinking part -- has to expend energy to not react to stimulus such as a luscious piece of calorie-laden pie. As one researcher says in the book, willpower is more than a metaphor: It's an expensive metabolic activity. That seems doubly unfair, doesn't it? You need to take in fuel (eat) to control your urge to eat.

Sometimes even the best-laid plans go wrong. If you're trying to lose weight and you expect to never falter and never get off track, well ... you're fooling yourself. It's a rare person indeed who can change his or her activity and eating habits and never slip a little bit from their new ways.

Many people who are trying to lose weight succumb to "all or nothing" thinking. As in "I screwed up on my diet this morning, so I might as well just quit."

Occasional slips are just a part of weight loss. Heck, they're a part of life. You may have a slice of cake at a birthday party. You may cave in to a second helping of your yummy dinner. You may skip your fitness routine for a day. And you know what? That's OK. The trick is to not let one little slip become a big slide. One small piece of cake isn't going to negate all of the other efforts you've made toward weight loss. One skipped day of exercise isn't going to throw off your whole routine. Just be sure to get right back on the wagon and keep moving forward. AOL Body has some tips to help you stick to your diet no matter what, including how to regroup after relapse.

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