If you have trouble getting a good night’s rest, the key to fixing it may be tracking your REM cycles. We all go through several in a night, and waking up in the middle of one (even the third or fourth cycle) will leave you feeling groggy and tired. There are several services that can help with this, including the Natural Rhythm Sleep Calculator. It calculates when you should plan to hit the hay or wake up, based on your natural rhythm. You can tell it that you want to sleep now and it will calculate what time in the AM will coincide with lighter sleep; or you can tell it when you absolutely must be up for work and it will give you the ideal bed time. Either way, using this tool will help you figure out the best, most efficient sleep for your body.