Why do you get a headache when sleeping for a long time? Shouldn't it be the other way around? However, everything is not so simple in our body. The fact is that long sleep implies a prolonged "squeezing" of blood vessels, including those that go to the brain. That unpleasant effect of long sleep is also promoted by the fact that in a horizontal position, the outflow of venous blood from the head is hampered. As a result, blood not enriched with oxygen is retained in the brain, and oxygen-rich blood comes in smaller volumes. Add to that the lack of energy. You've not eaten or drunk for 10-14 hours (or whatever length of time you chose for the long-awaited rest). So, it is not beneficial to let the body go to extremes. That's why 7-8 hours is still the ideal time interval for sleep.