Hi mate,
7 hours a night sounds great...when I have nothing to worry about and I don't have work I can easily sleep for 9-10 hours a day every day, but give me the stuff I can worry about and I can turn that into 4-5 hours a night some nights. In rare cases insomnia will send me to work on just an hour or two, but that's only three or four times a year that it'll get that bad.
One of my friends is a doctor, and he regularly builds up a sleep debt during the week and catches up on the weekend a bit like you are. Maybe try changing your routine - could you stay late in school to finish your lesson planning then get home a couple of hours later but with nothing to do apart from relaxing?
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