I got this from yahoo answers. This is pretty good for you people wanting to manage your sleep well. A really cool trick to helping your sleep is to learn about sleep cycles. (Sorry in advance if this gets long) A typical person goes through a full REM sleep cycle in 90 minutes (this can average +/- ~5-10 minutes a person). With this in mind, it is critical to make sure you plan your sleep schedule to allow yourself to wake up as close to the end of a cycle as possible. Therefore for most people the best amount of sleep to receive is 4.5 hour, 6 hours, 7.5, or 9. The next step is to learn how long it takes you to fall a sleep on average. This can average anywhere from 5 minutes to 2-3 hours for some people, so this is really something you'll have to figure out on your own. And this just comes with time, most people have a ballpark idea of how long it takes. So say it takes you about 30 minutes to fall asleep, and you know you need to get up at 8am. The best times to head to bed would be 10:30pm, Midnight, 1:30am, or 3:00am. The key to all of this is that it's actually more beneficial to get 7 and a half hours of sleep vs. 8. If you don't hit a sleep cycle well, say you get 8.5 hours of sleep, you will wake up in the middle of a REM cycle and actually be MORE tired vs. getting 6 hours of sleep and hitting the cycle right. I actually suggest to people if you know you need to get up at 8am, and you look at the clock and it's 12:30am, it's actually MORE beneficial to stay up until 1:30am so you can plan 30 minutes to fall asleep, and then get your 6 hours of sleep. The great thing about these sleep cycles is it can start working for some people within the first night or two of starting this. Some people don't see improved energy throughout the day for a week or two on the cycle, but the important thing to keep in mind is this is NOT dualistic. And by that i mean, it isn't important to always to doing this, and if you don't plan a sleep cycle for a day or two ( or a week/month) it doesn't set you back at all. If it's sunday afternoon and you want to sleep for 10-11 hours, go for it! Don't worry about always sleeping in 90 minute intervals. Hopefully I've explained this well, if you are curious about learning more you can always google 90 minute REM cycles I love discussing sleep cycles, lucid dreaming, and human health, so feel free to hit me up anytime. Hope this works well, and you start feeling better. I guess this explains why sometimes I only get 4-5 hours of sleep and when I wake up I don't feel too tired vs. times when I sleep for 7 hours and when I wake up I feel like shit.