You might want to research "sleep cycles". Apparently we go through cycles (lasting 90-120 minutes) as we sleep, and if you wake up in the middle of a cycle, you'll be more groggy and disoriented, and more tired, than if you woke up inbetween cycles. So sleeping only a few hours but waking up between cycles actually can make you feel more rested than if you slept a lot longer but woke up in between cycles. This is why sometimes, when you've gone to bed really late and then woken up really early, you actually wake more alert than if you'd slept 8 solid hours. You can experiment with different amounts of sleep and see what helps you most.
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