The Palaeolithic era ended 10k years ago.
Get as much sleep as you want, use sleepbot and/or fitbit to measure it and add your own quality ratings, grab the data and run it through R to plot a histogram for yourself. It's more scientific than assuming the past should dictate such things as "people may be meant to sleep".
FWIW I enjoy 7.2hr on average. Over 8 is nice but tends to be met by a sub-average duration the next night to compensate.