Cheat Day okay???

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I've been doing really well with eating healthy - high protein, low carbs, and almost no sugar. I also do cardio everyday and sporatic strength training. I started at 146lbs, and am down to 136lbs. My goal is a toned, muscular 130lbs.

I always run into a "hiccup" on Fridays. I'm a teacher, and we always have "Friday Treat." I overindulge, and then feel super guilty about it the rest of the day.

I go back to my healthy eating the next day, and continue my workouts.

My questions is: I've started embracing Friday Treat Day as a day I can enjoy some of the foods I've cut out, as a reward for doing well the rest of the week. If I'm careful not to go too crazy, am I hurting my diet my going over my calories once a week? (about 500 calories over!!!) My weight loss has slowed, and I'm trying to determine if I've just plateaued, or if it's my Treat Day...

(Although, I should mention I have a crazy sugar headache right now from reintroducing sugar to my diet after abstaining so long, ugh!!!)