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It seems like the last thing you want to do, but regular exercise is a great energy booster. Pack up the kids and take a brisk walk or even better take an hour all by yourself to do whatever kind of exercise you enjoy. Avoid lots of processed food and sugar, that gives you a pick up and then even lower energy low. Eat whole grains, fruits and veggies and drink lots of water. Try to make the kids nap at the same time so you can lay down too. Even with my oldest that does not nap, I still have him spend an hour quiet time in his room while his sister naps and I can recharge on my own. If it persists, you might check with your family doctor about anemia or depression, just a couple of things that come to mind that will make you feel very tired. Take care of yourself before you can take care of your kids! Good luck!
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