I clinch my teeth...how can I stop this?

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It doesn't happen all the time, but it lasts for about a week . I've tried those plastic mouth pieces, but they're very uncomfortable. I bite down so hard on my teeth that it makes it difficult to chew food afterwards. I try positioning my tongue to stop it, but that doesn't work too well. Does anyone else have this problem and do you have any suggestions on how to stop it?