Quite a few people complain about waking up and feeling tired even though they have actually ended up getting a full night’s sleep in their opinion. This comes from the assumption that you need eight hours of sleep every night, but in spite of the fact that this is the case the actual number is a bit more complicated than that if you think about it. It is important to note that it would be far more effective for you to wake up during non-REM sleep rather than to wake up during REM sleep just because it has been eight hours since your head hit the pillow.
The thing that you should take stock of here is that in order to increase your longevity by getting enough sleep you must first realize that your sleep is basically broken down into 1.5 hour cycles. This is how much time it takes for you to go into deep sleep and then come back up to light sleep. You should basically divide the amount of sleep that you are getting into 1.5. This will tell you how many cycles of sleep you are getting.
By this logic, sleeping eight hours will make it so that you would awaken as you are starting to drift back into REM sleep rather than light sleep. This is the main reason for so many people feeling groggy and being unable to wake up in the morning. Sleeping 7.5 hours or even 6 hours is going to be a lot better for you than getting more sleep than this. Remember that 7.5 hours is the sweet spot for you, and if you want to sleep more you should get 9 hours.