I don't know how many hours I need, but I do know that I need to reach the deeper type of sleep, which I haven't been able to do the last couple of weeks. Unless I reach the deeper sleep, 4 or 8 hours won't make a big difference.
Recently, I always wake up after 4 hours of sleep. Then, if I go back to sleep, it's another 4 hours. And it doesn't exactly help that I do some morning shifts at work, which means I have to get up at 4 am. And I'm a night person, so I usually end up getting 1-3 hours of sleep. Sometimes, I simply stay up until 4 am. Morning shifts are hell.