I decided to get used to a regular and quite ideal sleep schedule at the beginning of this week, (10 PM - 6 AM) and I managed to stick to it quite nicely.
The only problem was that I would always wake up at around 4:30 AM, then fall asleep again, and then feel tired when I woke up.
So this last night I thought “screw it” and went to bed right before 12 AM instead, and got up the exactly same time as before - and for some reason I felt super refreshed, not just “alert”, but seriously refreshed in a wonderful way - and I even woke up naturally, a moment before the alarm clock started to ring.
I think this is kind of strange, one could normally expect that I would need more sleep if I’m tired after 8 hours, but I was more awake after less sleep time instead…? 
It all depends on what stage of sleep you are in when you wake up.
For example, if you wake up during deep sleep, you’ll feel much more tired.
Wow, that was a fast reply. 
Yes I can see that this might be possible, but don’t you also normally have much more light REM sleep after around 6 hours or so?
I read an interesting study about this.
Stick to your schedule, your body loves consistency when it comes to your sleep cycles. But try this: Right before you decide to turn around and pass out for a glorious 8 hours of sleep, set an alarm on your phone or alarm clock for 2:40 A.M. When your alarm wakes you up try and stay awake for 10 minutes, after 10 short minutes you may go to sleep again and wake up at 6 a.m.
Studies show that 10 mins of wakefulness after 4 and a half hours of sleep increases your total REM sleep (the sleep stage that makes you feel rested, in fact scientists don’t even know why the other stages of sleep are there) during the night. The exstra 10 mins is just a personal choice that accounts for the time it takes you to fall asleep.
You WILL feel more rested when you wake up and this is a perfect tool for keeping an interesting dream journal or practicing various lucid dreaming induction techniques. You will also stop waking up at 4:30 a.m.
Let me know if this helps:)
p.s. to answer your question on the REM sleep, you have a period of REM sleep at the end of a 90 min sleep cycle, then you wake up very briefly and fall back asleep. The duration of REM sleep increases with each successive (90 min) sleep cycle. REM generally starts at roughly 10 mins for the first cycle to 30-45 mins for the final cycle of sleep.
p.p.s The amount of sleep needed to feel rested is influenced by a certain gene we all have, for some people this gene makes them need more sleep and for other people they need less sleep. The very possible but less likely answer to your question is: You only need 6 hours of sleep, not 8 hours. The needed 8 hour chunk of sleep every night is a myth.
I actually saw somewhere that humans in history, actually slept in two separate chunks of four hours each with a 30-minute to few hours pause between them, the modern conception of 8-hours per night straight, is a relatively new phenomenon.
Since it’s relevant: Polyphasic sleep.
I’m actually going to write an article for the forums when I get the chance. I’ll probably start work on it this weekend.
Like Mew said, I believe the cycle you’re in when you wake up greatly affects how you feel when you do. I’ve gotten up before with no more than four hours feeling superb, and yet there are nights when I get 8 and a half hours and wake up feeling miserable.