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There are quite a few things you can do to "reset" your sleep schedule, as shown by this WebMD article. However, be aware that due to your own circadian rhythms, that may not be the optimal pattern for you. Some of the suggestions include:
Bright lights - Use bright lights around you when you first get up.
Dim lights - Conversely, use dim lights in the evening.
Don't lay awake - If you tend to lay awake for a while, do something else before you go to bed.
Time meals - Use your meal times to help your body adapt to when to sleep. If you eat at irregular times, it can throw off your body.
Limit caffeine intake (Especially later in the day)
There are a few other suggestions in the article, especially relating to travel. One of the other suggestions is supplementing with melatonin, but as can be seen by this study review, many of the studies are not very well done, and a few directly refute melatonin being a sleep aid.
If you have trouble adjusting to a more traditional sleep schedule, you may want to look at biphasic sleeping (2, 4 hour sleep shifts) or contact a sleep center for help.
You should just stick to the new desired sleep routine and then accept that you'll have a jet lag for a some time. You may not sleep well the first few days, but you should force yourself to get up at the scheduled wake-up time. If you want to set your biological clock 6 hours back, then it may take a week before you're fully adapted to the new routine.
Make sure your bedroom is dark during sleeping time. After a few days into the new routine you should exercise, even if due to excessive sleepiness you don't feel like doing so. A potential problem a few days into the new routine can be that you have accumulated a sleep deficit due to not sleeping well a few days, but this will affect your brain far more than your body. If you give in to that by avoiding physical activity, then you may continue to sleep badly and the change to the new routine may take longer.
Of course, if you feel very sleepy, you should not exercise as fanatically as you are used to. Just start slowly (compared to your usual exercise routine), you may feel that during the exercise session the sleepiness goes away and stays away quite some time after you've finished. That's the desired effect, your body is then fully awake, and that allows you to sleep better at night.
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