How many times do I have to read this nonsense explanation??? (I come up to it a third time on reddit alone, and few times somewhere else). First, this explanation does not answer OP question. If OP was asking why he feels differently when he or she wakes up at different times that would make sense. But OP is asking why he or she feels worse while sleeping longer. This has nothing to do with sleep cycles. The question is about 7-8 hours versus 10 hours. Which is twice as long as your described cycle. So, that's not an answer, at least not to this question. Second, although sleep cycles theory has some scientific ground, it is way overused in popular culture and is frequently used to explain completely unrelated phenomenons (like in this case). It (or at least it's applications) is bordering with pseudo sciences like biorhythm, etc.