What is better for the body, to sleep 6 hours (4 deep sleep cycles) or to sleep 7 hours, but interrupting the last sleep cycle. I know I feel a lot better with 6, but am curious as to its medical consequences. Does the brain have a chance to repair/clean itself properly with this lesser amount?
The thing about sleep is that there is no "one size fits all". Think about it, if we are all biochemical different, then why would we all need the same amount of sleep? Why would all of our sleep cycles last the same amount of time?
I was once a severely insomniatic person. I would worry about not getting that perfect eight hours of sleep that everyone talks about being so important, and despite trying my hardest to get eight hours of sleep it would never happen. I would either get no sleep at all, or I'd sleep in and get over twelve hours of sleep.
One day I was talking with my professor who specialized in sleep, and he suggested that I aim for four or five hours of sleep and to see how I start to feel. He also said to pick a specific time that I want to wake up everyday. Oddly enough, it fixed my problems. I can now go an entire day without thinking about how tired and fatigued I am.
So to answer your question, it does not matter. Wake up when you need to wake up, and go to bed when you are feeling tired. If you find that you feel good throughout the day having only slept 6 hours, then only sleep six hours. If you feel tired throughout the day, then you might need more sleep.
Your body will "tell" you if you need more or less sleep. But you need to listen to it.
What I'm saying plays in to what Raja's comment is getting at, "is sleep cycle theory correct"?
Neither Sleep cycles do not have a set length.
sleep for 7 hours every night and you won't interrupt a sleep cycle. 6 hours may be fine for a person but it is unlikely.
You feel tired when you interrupt a sleep cycle becasue many of the sleep inducing hormones are still present in your system. http://apsychoserver.psych.arizona.edu/jjbareprints/psyc501a/readings/Carskadon%20Dement%202011.pdf
If you are sleeping different lengths every night (or say sleeping longer on the weekend) that will also make you feel tired. Normally your body adjusts the length of your sleep cycle to how long it predicts you will sleep for, if that is constantly changing however it cannot do this and you often wake up in the middle of a cycle making you feel tired. Sleep the same length of time for ~14 days and you will see your sleep cycle stabilize.